This is a precursor to:

* #536

Scala 2.13.11 added a warning that implicit definitions [should](
https://nrinaudo.github.io/scala-best-practices/tricky_behaviours/type_implicits.html) have an explicit type (because implicit resolution is already complicated enough for the compiler, and things like file-order can actually make a difference to whether implicit scopes are correctly searched: scala/bug#8697 (comment)):

* scala/scala#10083

The warning looks like this:

```
[error] ~/code/presence-indicator/app/autoscaling/Notification.scala:7:16: Implicit definition should have explicit type (inferred play.api.libs.json.Reads[autoscaling.Notification]) [quickfixable]
[error]   implicit val jsonReads = Json.reads[Notification]
[error]                ^
```

...this prepares us for Scala 3, where the explicit type is _required_.

More widely, beyond implicit definitions, in **library** code, the best practice is to always add an explicit type to all your **public** members, even when you're happy with what's being inferred - otherwise you can unintentionally break binary-compatibility just by changing the _implementation_ of a field:

* https://docs.scala-lang.org/scala3/guides/migration/incompat-type-inference.html
* https://nrinaudo.github.io/scala-best-practices/binary_compat/explicit_type_annotations.html
* https://scalacenter.github.io/scalafix/docs/rules/ExplicitResultTypes.html

## Automatically fixing this code issue

Scalafix actually does a better job than `-quickfix` for this particular task, because it adds imports if it needs to, so that you end up with this in your code:

```
implicit val jsonReads Reads[Notification] = Json.reads[Notification]
```

...rather than something like:

```
implicit val jsonReads Reads[com.gu.blah.foo.bar.Notification] = Json.reads[Notification]
```

### Fixing while still on Scala 2.12 - use Scalafix

In this commit, we're only trying to fix the implicit definitions, so I've added this in a new `.scalafix.conf` config file:

```
ExplicitResultTypes.onlyImplicits = true
```

The Scalafix rule needs to be run while the project is still on Scala 2.12, not Scala 2.13
(otherwise sbt will say: "Error downloading ch.epfl.scala:sbt-scalafix;sbtVersion=1.0;scalaVersion=2.13:0.13.0").

Once the Scalafix plugin is made available to sbt (by adding `addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.13.0")`
to either `project/plugins.sbt` or `~/.sbt/1.0/plugins.sbt`), you can run these commands on the sbt prompt to automatically generate the changes in this PR:

```
scalafixEnable
scalafixAll ExplicitResultTypes
```

This is a precursor to:

* #536

Scala 2.13.11 added a warning that implicit definitions [should](
https://nrinaudo.github.io/scala-best-practices/tricky_behaviours/type_implicits.html) have an explicit type (because implicit resolution is already complicated enough for the compiler, and things like file-order can actually make a difference to whether implicit scopes are correctly searched: scala/bug#8697 (comment)):

* scala/scala#10083

The warning looks like this:

```
[error] ~/code/presence-indicator/app/autoscaling/Notification.scala:7:16: Implicit definition should have explicit type (inferred play.api.libs.json.Reads[autoscaling.Notification]) [quickfixable]
[error]   implicit val jsonReads = Json.reads[Notification]
[error]                ^
```

...this prepares us for Scala 3, where the explicit type is _required_.

More widely, beyond implicit definitions, in **library** code, the best practice is to always add an explicit type to all your **public** members, even when you're happy with what's being inferred - otherwise you can unintentionally break binary-compatibility just by changing the _implementation_ of a field:

* https://docs.scala-lang.org/scala3/guides/migration/incompat-type-inference.html
* https://nrinaudo.github.io/scala-best-practices/binary_compat/explicit_type_annotations.html
* https://scalacenter.github.io/scalafix/docs/rules/ExplicitResultTypes.html

## Automatically fixing this code issue

Scalafix actually does a better job than `-quickfix` for this particular task, because it adds imports if it needs to, so that you end up with this in your code:

```
implicit val jsonReads Reads[Notification] = Json.reads[Notification]
```

...rather than something like:

```
implicit val jsonReads Reads[com.gu.blah.foo.bar.Notification] = Json.reads[Notification]
```

### Fixing while still on Scala 2.12 - use Scalafix

In this commit, we're only trying to fix the implicit definitions, so I've added this in a new `.scalafix.conf` config file:

```
ExplicitResultTypes.onlyImplicits = true
```

The Scalafix rule needs to be run while the project is still on Scala 2.12, not Scala 2.13
(otherwise sbt will say: "Error downloading ch.epfl.scala:sbt-scalafix;sbtVersion=1.0;scalaVersion=2.13:0.13.0").

Once the Scalafix plugin is made available to sbt (by adding `addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.13.0")`
to either `project/plugins.sbt` or `~/.sbt/1.0/plugins.sbt`), you can run these commands on the sbt prompt to automatically generate the changes in this PR:

```
scalafixEnable
scalafixAll ExplicitResultTypes
```