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
Adds first translations to source code and sets up transifex config
file. Pull more translations later with:
cd briar-android
tx pull --mode=developer
Closes#448