Speed up initial message loading

Start loading messages before we have finished loading
information about message groups, which usually takes
multiple seconds and longer than actually loading the list
of messages. Because the group workflow state selector is
updated when messages are loaded, I had to introduce
mw.translate.getMessageGroup that asynchronously returns
information about the group.

Also note that mw.translate.getGroup also exists, but it
is added by ext.translate.groupselector and is not async.
Perhaps we can get rid of it later.

Another thing we could is to turn the list of message
groups returned by the API into an object indexed by the
message group id, making lookups faster and simpler.

Change-Id: I03467c02985cf27c5be1192ef5ae7ccd2cd7159a
Merge "Speed up initial message loading"