Add massmessage-sender user group to enwiki

This patch adds the 'massmessage-sender' user group
to the English Wikipedia, 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: 58962
Change-Id: I20acaa34171f1040052a5330caab535aa3f9aaa8