Don't require users be logged in to access the Logout controller, so users with no Spaces can log out

Summary:
Fixes T13310. Use cases in the form "users with no access to any spaces can not <do things>" are generally unsupported (that is, we consider this to mean that the install is misconfigured), but "log out" is a somewhat more reasonable sort of thing to do and easy to support.

Drop the requirement that users be logged in to access the Logout controller. This skips the check for access to any Spaces and allows users with no Spaces to log out.

For users who are already logged out, this just redirects home with no effect.

Test Plan:

• As a user with access to no Spaces, logged out. (Before: error; after: worked).
• As a logged-out user, logged out (was redirected).
• As a normal user, logged out (normal logout).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13310

Differential Revision: https://secure.phabricator.com/D20578