Use Roboto Light, thinner lines between tiles, slightly smaller letters

For more than 4 members in a conversation, the fourth tile now contains
an ellipsis to indicate this

Notifications now have pseudotransparent icons, icons are generated in
the correct size.

transmitting files between two conversations works. no error handling and no ui on the receiving end

Now, MUCs have icons with up to 4 tiles representing MUC members,
similar to the gmail app.

Some caveats:
As of now, no ordering is imposed on the members, we simply pick the
first (up to) 4 returned by the MucOptions object. This could be done
better, e.g. by picking more important members first (based on
affil/role), or even going so far as to order them by how recently they
sent a message in the MUC.
Also, the code generating the tiles is really messy right now (tons of
copy and paste). It seems to work though. I will clean this up soon.

Use polymorphism to avoid dynamic dispatch based on nulled parameters.
Next up: Prettier MUC icons

Normalize the interface, reduce code duplication

changed behaviour of expecption handler to check if accounts are working before removing the stacktrace

Switch to FULL_SYNC to make prosody happy