Let users review their own account activity logs

Summary:
Ref T4398. This adds a settings panel for account activity so users can review activity on their own account. Some goals are:

  - Make it easier for us to develop and support auth and credential information, see T4398. This is the primary driver.
  - Make it easier for users to understand and review auth and credential information (see T4842 for an example -- this isn't there yet, but builds toward it).
  - Improve user confidence in security by making logging more apparent and accessible.

Minor corresponding changes:

  - Entering and exiting hisec mode is now logged.
  - This, sessions, and OAuth authorizations have moved to a new "Sessions and Logs" area, since "Authentication" was getting huge.

Test Plan:
  - Viewed new panel.
  - Viewed old UI.
  - Entered/exited hisec and got prompted.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4398

Differential Revision: https://secure.phabricator.com/D8871