Allow users to associate SSH Public Keys with their accounts

Summary:
With the sshd-vcs thing I hacked together, this will enable Phabricator to host
repositories without requiring users to have SSH accounts.

I also fixed "subporjects" and added an explicit ENGINE to it.

Test Plan: Created, edited and deleted public keys. Attempted to add the same
public key twice. Attempted to add invalid and unnamed public keys.
Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran, cadamo, codeblock
CC: aran, epriestley
Differential Revision: 711