group context handling fixes and localization

This is the non-UI work.

* Localized many group update strings
* Allow for group context messages to unset avatar

From a technical perspective, it seems more correct. If we're to assume group updates contain the full state of the group, then the update not having an avatar means there isn't an avatar in the group.

From a UX perspective, it's nice to be able to unset things that you've set. Other messenger clients offer the same.