API: Fixes for AuthManager

API: Fixes for AuthManager

* Set API response metadata on the AuthenticationRequest metadata so it
  is output as an assoc generally.
* Remove the 'image' field in AuthenticationRequest::getFieldInfo()'s
  response, since we ended up not using it anywhere.
* Make it so meta=authmanagerinfo can be used on private wikis without
  logging in, so action=clientlogin can be used to log in.
** This generalizes the exception for meta=tokens that was added in
   I83dafb030.
* ApiAuthManagerInfo needs the "messageformat" parameter.
* ApiAuthManagerInfo shouldn't be publically cached, since the responses
  vary depending on session state.

Change-Id: Iea5ddb4ef9febed18f16a7ae8314968026f39148