xmpp urls, languages, and misc fixes

This is my work over the past week, here is the crux of it:

* sorted out `xmpp:` and `im:` URLs to follow the related RFCs and coordinated with @chrisballinger / @chatsecure about a format that will work on both platforms, more info here: https://dev.guardianproject.info/issues/4092

* updated the Languages chooser in Settings to be based on whatever languages are installed.  It also gets the name of that language from the system instead of manually maintained list.

* misc related fixes that I encountered along the way