Validate Arcanist install-certificate URIs more carefully

Summary: Fixes T11222. This was lazy-future-proofed for Conduit SSH support, but users are boundlessly creative. Check protocols explicitly.

Test Plan:

$ arc install-certificate a.b:1/
Usage Exception: Server URI "a.b:1/" must include the "http" or "https" protocol. It should be in the form "https://phabricator.example.com/".

• Also went through a successful workflow with a URI in the form provided in the example.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11222

Differential Revision: https://secure.phabricator.com/D16188