fix(groups): forces content accessibility to members_only for invisible groups

- [x] When user selects "Group members only" for group visibility, javascript sets the group content accessibility field to "Members only" and disables it.

- [x] For existing groups the above is done also on server side.

- [x] When editing an existing public group, a warning is added under the group content accessibility input saying that changing the setting won't change the access permission of existing group content.

This should pretty much finish fixing #6142


To-do before merging:
- [x] switch js to AMD
- [x] import Elgg (`var elgg = require("elgg")`)
- [x] consider namespacing this module ("elgg/groups/edit")