Group pages should not show Messages tab to unauthorised users

[erlend_sh (Erlend Sogge Heggen)]

There’s no point in showing this tab at all if the user doesn’t have the necessary privileges to view the contents in the stream:

[codinghorror (Jeff Atwood)]

Sure we should fix this @techapj

[techAPJ (Arpit Jalan)]

Done via:

https://github.com/discourse/discourse/commit/c626558d36c3e50de7abc332fd7bf55c7829cbd6