R1:4f8340c05f2b

Restore the "Log In" menubar action

Summary:
See <https://discourse.phabricator-community.org/t/activation-link-in-welcome-mail-only-works-if-new-user-isnt-semi-logged-in/740/7>.

In T13024, I rewrote the main menu bar to hide potentially sensitive items (like notification and message counts and saved search filters) until users fully log in.

However, the "Log In" item got caught in this too. For clarity, rename `shouldAllowPartialSessions()` to `shouldRequireFullSession()` (since logged-out users don't…