Add support for connecting remotely to the MyBB forum

[lairdshaw]

By creating an additional plugin for the MyBB forum, authentication could take place over an HTTP(S) connection using an API. This would eliminate the restriction for the MyBB forum of a MySQL/MariaDB, as well as any need to assume a particular MyBB password hashing mechanism (I've been informed that the DVZ Hash plugin allows the default password hashing mechanism to be changed). Through the use of a shared secret key, the use of the API would be restricted to the trusted MediaWiki installation.