In the home timeline statuses might be present because the user follows
one or more of the hashtags in the post. In other timelines the user
might still want to see that a post contains one or more of their
followed hashtags.

So

- Highlight hashtags the user follows in all posts, by making them bold.
- In the home timeline, show the first followed hashtag in the info bar.
Tapping the hashtag in the bar opens the timeline for that hashtag.

To do this:

- Fetch the user's list of followed tags when switching accounts, and
attach it to `PachliAccount`.
- Add the `following` property to the hashtags on statuses received
from timeline queries.
- Use the new `FollowedHashtagSpan` to mark up hashtags the user is
following.
- If the statusInfo is empty and the status contains a followed
hashtag then show this in the statusInfo.

Fixes pachli#1923