Keep track of which RSS entries have been seen
This is done by remembering the time of the latest entry.
All entries newer than that are considered new and will be posted.
Closes#485
See merge request !249
Implement background task for fetching RSS feeds
* Implemented in briar-core as a `ScheduledExecutorService`
that gets started when the app starts
* The briar-api has a `FeedManager` interface
that the UI can use to register and unregister feeds
* In this first iteration, feeds are fetched via HTTP(S), not Tor
Closes#484
See merge request !247
* Implemented in briar-core as a `ScheduledExecutorService`
that gets started when the app starts
* The briar-api has a `FeedManager` interface
that the UI can use to register and unregister feeds
* In this first iteration, feeds are fetched via HTTP(S), not Tor
Closes#484
UX improvements for crash reports and feedback
(The background color of the first screenshot is white, but I now changed it to use the same background color like everywhere)

Closes#335
See merge request !246
The old behaviour was a leftover from the days of limited retention periods. The new behaviour will interact better with dependencies and message queues.
Pause Periodic List Refresh when View is not Visible
This MR also sneaks in a second commit which stops the cevron in the forum list to sometimes lose its blue color. I noticed this when testing this MR with short refresh intervals. This is not as nice of specifying the color as a tint, but unfortunately tints are not yet ready for primetime on Android.
Closes#553
See merge request !259
Show notifications for new blog posts
This also adds a setting allowing people to turn blog post notifications off.
As instructed, this does not yet improve upon the current notification mechanism, but rather follows it, leaving the improvements for #289.
Closes#488
See merge request !258
Show Personal Blog When Clicking On Post in Combined Blog Feed
This is your own personal blog when clicking on your own posts. Note how you can also write posts from that screen and no trust indicator is shown for the posts:

This is somebody else's blog on a dedicated screen after clicking on a blog post:

Closes#415
See merge request !256
Use Client Layer Events in ContactListFragment
This prevents trying to access the same group metadata in different groups.
Also, the conversation does not need to be reloaded once introduction messages arrive.
Closes#535
See merge request !254
Remove PersistentData classes as they aren't thread-safe
This MR depends on !244 and does not claim to resolve *all* issues with the ForumActivity.
Closes#544
See merge request !253
Introduce client layer events for forums
The forum UI depended on sync layer events such as `MessageStateChangedEvent`.
Now, the forum client broadcasts its own high-level event (`ForumPostReceivedEvent`)
with the information the UI needs (`ForumPostHeader`).
Closes#310
See merge request !244
Database queries for metadata only returned it for messages that were delivered already.
However, there are cases (e.g. a pending message needs to be delivered) where
the validator needs to retrieve the metadata from the database.
For these cases, a special database query has been introduced.
The forum UI depended on sync layer events such as MessageStateChangedEvent.
Now, the forum client broadcasts its own high-level event (`ForumPostReceivedEvent`)
with the information the UI needs (`ForumPostHeader`).
Closes#310
This prevents trying to access the same group metadata in different groups.
Also, the conversation does not need to be reloaded once introduction messages arrive.
Closes#535