Add user's email address to user-specific urls [MJ-350]

Redirect to 404 page if the user tries to access another user's resources
Log out existing user if the url token doesn't match their account

Adds the `user_is_allowed` decorator to do this. It should be generic enough to use for profiles and employer dashboards as well as saved searches.

This looks like a lot of changes, but most of it is just adding the new argument to saved search urls.

@michaelgzoller I was supposed to work with you on this, but I didn't read that part until just now. Hope you don't mind.

All tests pass.