[Bugfix] Joining helpdesk team

[PandaWhisperer]

Fixes

• Allow "Join [Teamname]" button on /team/:id page to join the Helpdesk team
• Show error message when trying to join any other team
• Add specs for JoinController

[Montana]

Checked with Rollbar Scope, and approved. Good collab.

scope = {request: request_data,
         person: person_data,
         context: context_data
}
Rollbar.scoped(scope) do
  begin
    @app.call(env)
  rescue Exception => exception
    # ...
  end
end