Be more strict about "Location:" redirects

Summary:
Via HackerOne. Chrome (at least) interprets backslashes like forward slashes, so a redirect to "/\evil.com" is the same as a redirect to "//evil.com".

  - Reject local URIs with backslashes (we never generate these).
  - Fully-qualify all "Location:" redirects.
  - Require external redirects to be marked explicitly.

Test Plan:
  - Expanded existing test coverage.
  - Verified that neither Diffusion nor Phriction can generate URIs with backslashes (they are escaped in Diffusion, and removed by slugging in Phriction).
  - Logged in with Facebook (OAuth2 submits a form to the external site, and isn't affected) and Twitter (OAuth1 redirects, and is affected).
  - Went through some local redirects (login, save-an-object).
  - Verified file still work.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10291