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.
What is the purpose of setting the `destination` variable if it only used once, in the `redirect_to`, within this action? It strikes me as odd for two reasons:

- it does not get set near the `redirect_to` so when reading through the code to see where it goes, it does not seem intention revealing
- I do not think I have seen this anywhere else in Supermarket (could totes be wrong), which makes me wonder why?