Use better secrets in generating account tokens

Summary:
When we generate account tokens for CSRF keys and email verification, one of the inputs we use is the user's password hash. Users won't always have a password hash, so this is a weak input to key generation. This also couples CSRF weirdly with auth concerns.

Instead, give users a dedicated secret for use in token generation which is used only for this purpose.

Test Plan:
  - Ran upgrade scripts.
  - Verified all users got new secrets.
  - Created a new user.
  - Verified they got a secret.
  - Submitted CSRF'd forms, they worked.
  - Adjusted the CSRF token and submitted CSRF'd forms, verified they don't work.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8748