R1:cedb0c045ad7

Lock down accepted next URI values for redirect after login

Summary:
I locked this down a little bit recently, but make
double-extra-super-sure that we aren't sending the user anywhere suspicious or
open-redirecty. This also locks down protocol-relative URIs (//evil.com/path)
although I don't think any browsers do bad stuff with them in this context, and
header injection URIs (although I don't think any of the modern PHP runtimes are
vulnerable).

Test Plan:
- Ran tests.
- Hit redirect page with valid…