R1:5b1d9c935a90

After writing "next_uri", don't write it again for a while

Summary:
Fixes T3793. There's a lot of history here, see D4012, T2102. Basically, the problem is that things used to work like this:

- User is logged out and accesses `/xyz/`. After they login, we'd like to send them back to `/xyz/`, so we set a `next_uri` cookie.
- User's browser has a bunch of extensions and now makes a ton of requests for stuff that doesn't exist, like `humans.txt` and `apple-touch-icon.png`. We can't distinguish between…