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This implements very basic and simple empty state messages for the
current features.

Closes #327

The adapter of the `ForumListFragment` has been changed into a
`BriarRecyclerView` and all its code has been adapted and simplified
accordingly.

All UI of the forum list is now defined in XML layouts.

It is a common pattern to have a list with an empty view and a progress bar.
This commit introduces a custom BriarRecyclerView and uses it for the
contact list.

No more manually hiding and showing empty views and progress bars is
necessary when using the new BriarRecyclerView instead of RecyclerView.

Please note that this conflicts with !44 at the moment and needs to be
implemented for !36 once merged.

Closes #198