Create a new Conduit API method to lookup Phabricator accounts by MediaWiki user name for symmetry with the user.ldapquery method that @mmodell made and to support user who have Phabricator accounts linked only to their SUL credentials.

Proposed name: user.mediawikiquery
Parameters:

• names: list of MediaWiki usernames
• offset: offset to start results from (optional)
• limit: max results (optional)

Response:
Same as user.ldapquery but with a mediawiki_username field for each result rather than an ldap_username field.