labhub.py: Alert when @mention is not in room

[abhishalya]

The fix introduces removal of is_room_member function and instead uses callback_message to warn whenever @mention is not in room.

Closes https://github.com/coala/corobo/issues/603

Reviewers Checklist

• [x] Appropriate logging is done.
• [x] Appropriate error responses.
• [x] Handle every possible exception.
• [x] Make sure there is a docstring in the command functions. Hint: Lookout for botcmd and re_botcmd decorators.
• [x] See that 100% coverage is there.
• [x] See to it that mocking is not done where it is not necessary.

[abhishalya]

Obviously the tests will fail as I have removed the is_room_member function.

[abhishalya]

@jayvdb Can you please check the warning message so that I can make changes to the tests?

[jayvdb]

tests need to be fixed.

Shouldn't that be a new issue?

No. Fixing/adding unit tests are an integral part of changing code.

[abhishalya]

Blocked until https://github.com/errbotio/err-backend-gitter/issues/38 is solved.