Admins can respond to requests to join their organization

:fork_and_knife: 

In short: the links in the emails that admins get following #606 now work.

Both accepting and declining a request result in a `ContributorRequestResponse` record being created; the value of the `decision` field of that response differentiates the two types of responses. When the request is accepted, a `Contributor` record is created for the requesting user so that they are officially a contributor on behalf of the organization. Whether or not the requesting user is accepted, they will receive an email informing them of the admins' decision.
At first glance, it looks like other emails are using punctuation unless they are title-ized. I would suggest adding a "." at the end of this for the sake of consistency. Same goes for the text email.