Now with 96% test coverage.

Related to #2.

Related to #26

Related to #50.

Generated with the Open Age Ratings Service:
https://hughsie.github.io/oars/index.html

That jar got build from the most recent commit

briar@f61e2b39

of the merge request adding the `lastChatActivityKey`

briar!1242

This now validates with
```
appstream-util validate app.briar.gtk.xml.in
```

This wasn't possible before because my version of flatpak was buggy.

Fixes #46.

Fixes #45.

Fixes #44

This reverts commit 2c5d71f0.

This is a change not-yet upstreamed but currently to be reviewed at
briar!1242.

Before, messages from all chats were added to the current chat.

Also scrolls to bottom now after adding a message async.

Before, there was no indication to which day a message belongs.