helpers for context switcher

cc @fancyremarker

`return_to_context_url` is used to define where the user should return to after switching their `current_organization` context.  The use case here is that I am looking at an `Apps#show` that belongs to Org A and then switch my context to Org B.  It would be confusing to remain on the `Apps#show` for Org A, so we redirect to `Apps#index`.
Also, should it be `root_url`? It seems that if a user is on, say, `RolesController`, this could result in a 403? Or no?