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