Provide an activity log for login and administrative actions

Summary: This isn't complete, but I figured I'd ship it for review while it's still smallish.

Provide an activity log for high-level system actions (logins, admin actions). This basically allows two things to happen:

• The log itself is useful if there are shenanigans.
• Password login can check it and start CAPTCHA'ing users after a few failed attempts.

I'm going to change how the admin stuff works a little bit too, since right now you can make someone an agent, grab their certificate, revert them back to a normal user, and then act on their behalf over Conduit. This is a little silly, I'm going to move "agent" to the create workflow instead. I'll also add a confirm/email step to the administrative password reset flow.

Test Plan: Took various administrative and non-administrative actions, they appeared in the logs. Filtered the logs in a bunch of different ways.

Reviewers: jungejason, tuomaspelkonen, aran

CC:

Differential Revision: 302