Fix the file names for locale strings

Two things:
 - The grunt script was checking for the wrong .po name, and thus wasn't generating
messages.json.
 - i18n-abide module use on the server is hardcoded to look for messages.json,
   rather than server.json.

So, convert server.po to messages.json and change the client code to look for
client.json instead.
remove duplicate config entries
Merge pull request #584 from mozilla/fix-json-strings-filename

Fix the file names for locale strings

This looks great, thanks @zaach!