Fix extension functionality and backwards compatibility

This updates the CA certificate and login endpoint to match Mozilla.
Also changes some things to make the extension backwards-compatible
with version 1.19 of MediaWiki.

Finally, this fixes a few minor bugs:
* Infinite looping when authentication is unsuccessful
* Non-internationalized error messages

Bug: 55975
Change-Id: If61cf139256e411440eb5a327f32f37bd6dfb962