"arc install-certificate", server-side components

Summary:
Provides a new workflow for making it non-horrible to install certificates.
Basically you run "arc install-certificate" and then copy/paste a short token
off a webpage and it does the ~/.arcrc edits for you.

Test Plan:
Installed certificates, used bad tokens, hit rate limiting.

Reviewed By: aran
Reviewers: aran, jungejason, tuomaspelkonen
CC: aran
Differential Revision: 460