Store repository credentials with repositories

Summary:
Move toward storing credentials in configuration so it's easier to get the
daemons working. This should eventually solve all the key juggling junk you have
to do right now.

This only gets us part of the way to actually using these credentials in the
daemons since I have to go swap everything for $repository->execBlah().

I tried to write a web "Test Connection" button but it was too much of a mess to
get git to work since git doesn't give you access to its SSH command and SSH has
a bunch of interactive prompts which you can't really do anything about without
it or a bunch of ~/.ssh/config editing. This is what Git recommends:

https://git.wiki.kernel.org/index.php/GitFaq#How_do_I_specify_what_ssh_key_git_should_use.3F

..but it's not a great match for this use case.

Test Plan:

• Only partial.
• Ran "test_connection.php" on a Git repo with and without SSH, and with and

without valid credentials. This part works properly.

• Ran "test_connection.php" on a public SVN repo, but I don't have private or

WEBDAV repos set up at the moment.

• Mercurial doesn't work yet.
• Daemons haven't been converted yet.

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, abdul, nmalcolm, epriestley, jungejason

Differential Revision: 888