Add a MassMessage-related user group on Meta

This patch adds a 'massmessage-sender' user group
on Meta, with the extension's only user right, ie.
'massmessage'; sysops will be able to add and
remove users from this group.

See bug for URL with on-wiki consensus for the
creation of this new user group.

Bug: 57611
Change-Id: Id9c7321b8586fab1ebc8a1bf0888adc93ff37e2f