Redirect after login always goes to home page
When a user logs in at one of the login forms brought up by a vote arrow, after their successful login they are redirected to the home page, instead of the post they were trying to update. The redirect should send them back to where they were. This used to work with the old login system, but got overlooked when we replaced it.
We should check that the target of the redirect is valid to avoid creating an open redirect. Since the redirect is handled in the front end, feeding the redirect to the front-end router to make sure it's something we recognize is a possible strategy for that.