Refactor user settings

Summary:
I want to do two things here:

• Add SSH Keys
• Move "Preferences" into this panel

But this controller was pretty gigantic and messy. Split it apart and use
delegation instead.

There are no functional changes. I changed some of the conduit certificate text
to simplify it since no one should need to go through that workflow anymore,
given the existence of "arc install-certificate".

Test Plan:

• Edited realname, including attempting to remove it.
• Edited profile picture.
• Edited timezone.
• Edited email, including attempting to remove it.
• Regenerated condiut certificate.
• Linked and unlinked an OAuth account.

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 688