## [SIEM] Overview page feedback

Implements feedback and fixes to the Overview page

### Overview (default theme)

![01-overview-default-theme](https://user-images.githubusercontent.com/4459398/73315509-899c5500-41ed-11ea-9949-82853dd4ba59.png)

### Overview (dark theme)

![02-overview-dark-theme](https://user-images.githubusercontent.com/4459398/73315527-902acc80-41ed-11ea-9701-6a2c5fa40cce.png)

## Highlights

* The new order of widgets is Signals, Alerts, Events, Host Events, Network events, per elastic/siem-team#494

* Changed the default `External alerts count` `Stack by` to `event.module` elastic/siem-team#491

*  Added `event.module` to the `Events count` histogram elastic/siem-team#491

* Widget titles will no longer include the currently selected `Stack by option`. The widgets will use the same static title text that appears on the other pages (i.e.. `Signals count`, `External alerts count`, and `Events count`) elastic/siem-team#491

* The `Signals count` includes a `Stack by` that defaults to `signal.rule.threat.tatic.name`

* Standardized on a 300px widget height for all histograms in the app (thanks @MichaelMarcialis for paring on this!)

* The `Open as duplicate timeline` action is `Recent timelines` is now only shown when hovering over a recent timeline

## Loading States

* The `Recent timelines` and `Security news` widgets now use the horizontal bar loading indicator

* The `Host events` and `Network events` widgets now use the horizontal bar loading indicator

* The `Host events` and `Network events` Showing _n_ events subtitles are now hidden on initial load

* The counts in the `Host events` and `Network events` Showing _n_ events subtitles are now hidden on initial load

* We no longer hide some histogram subtitles after initial load, to prevent shifting of content when a user makes a `Stack by` selection

## News Feed Error State

![news-feed-error-state](https://user-images.githubusercontent.com/4459398/73316060-1e538280-41ef-11ea-83f5-b8d6e9fa3741.png)

* Fixed an issue where the `Security news` header was hidden when an invalid URL is configured

* Added a space between the word `via` and the `SIEM advanced settings` link

* Removed the capital “N” from "News" in the error message

## Misc Visual Changes

* Fixed text truncation of the `Severity` column in the `Detections` page's `Signals` table

* Added the “showing” subtitle to the `Signals count` histogram on the Detections page

* Increased the `Stack by` histogram selector and the `View signals | alerts | events' buttons from 8 to 24px

* Tweaked the border rendering in the Overview `Host Events` and `Network events` widget headers

* Added 8px of spacing between the Overview `Host Events` and `Network events` widget accordion headers and their contents

* Fixed an issue where the `Host events` and `Networ events` widgets didn't render in ie11 elastic/siem-team#499

## Non-Visual Fixes

* Removed an incorrect usage of `usememo`

* Removed the placeholder client-side username query from `x-pack/legacy/plugins/siem/public/components/recent_timelines/index.tsx`

* Updated the query of the Overview `Host events` widget to filter by "host.name exists"

* Updated the query of the Overview `Network events` widget to filter by "source.ip exists or destination.ip : exists"