This commit refactors the code for sharing forums,
so it can be used for sharing blogs as well.
It does not yet include code for responding to blog invitations.
* 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
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
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
Micro Blogs UI
**Attention:** This MR includes several other commits which are supposed to end up in separate MRs. I suggest that you review **per commit**. Once the first two commits have green light, I can split out the other commits into other MRs. This way I don't have to work myself through a long rebase chain every time I make a change to the bottom MR.
This MR is full of commits that introduce features that we will not be using initially. The last commit implements the Micro Blogs UI on top of the framework the first commits establish and hides/disables all future features for now.
I suggest we merge this as is and clean things up later when we have a clearer idea what features we will be doing eventually.





Closes#436
See merge request !214
Make it clearer who will be introduced
Show contact names under avatars before sending introduction to make it clearer who will be introduced.

Closes#356
See merge request !242
Show all Forum Invitations, not only the first one to prevent cut-offs and forks
This MR solves the problems outlined in #461 by always showing forum invitations, even if the user already subscribed to the forum.
Available Forums have been renamed to Forum Invitations in the UI:


Closes#461
See merge request !241