Add OTRS-member group to enwiki

Add OTRS-member to enwiki, addable and removable by admins. Has read as a
dummy right just so it exists, as its purpose is just for marking. The
name is the same as the one used in Extension:WikimediaMessages, so the
messages will already be set up.

Bug: 70386
Change-Id: If39a7bd5c9ed2e8a51e3446756ad3c28bb67b878