added join workspace button if policy allows self joining

I see several issues to use that decorator, first it uses Forbidden exception, which returns 403 http status, while it should be 405, method not allowed, but end user normally don't see this, so doesn't matter that much. Also, in my opinion, 403 error somewhat implies that with more powerful privileges I should be able to do that request.

However, what matters is the representation of the error, and the end user gets an error page, telling that there was an error, which looks like, that server has crashed on that request, with no indication, that the user did something wrong.