Hardcode the bot's username as $wgMassMessageAccountUsername

Hooks were added to add the username to $wgReservedUsernames,
and to prevent the account from being renamed.

There are various issues with using a message, letting local
admins override it, etc, so it is just easier to set it as
a global variable.

Change-Id: I4e95ef97d693ba62d69758ac96401064e1b0f097
Merge "Hardcode the bot's username as $wgMassMessageAccountUsername"