Restrict groups drop-down to only groups allowed in access restrictions

When an activity uses groups, and restricts access to a subset of the available groups, the groups dropdown on the activity view should only show the groups that are allowed.

Scenario:

• A Course with visible groups Alpha, Bravo, Charlie, and Delta.
• A Forum allowing access to groups Alpha and Bravo.
• A student, member of groups Alpha and Delta.

Expected behaviour: The student can only see Groups Alpha and Bravo in the group drop-down, and is only able to post as group Alpha.

Current behaviour: The student can see Alpha, Bravo, Charlie, and Delta in the group drop-down, and can select Delta (or has it recalled from previously selecting it) and post to the forum as Delta.