Don't implement SVN over HTTP

Summary:
Ref T2230. As far as I can tell, getting SVN working over HTTP is incredibly complicated. It's all DAV-based and doesn't appear to have any kind of binary we can just execute and pass requests through to. Don't support it for now.

• Disable it in the UI.
• Make sure all the error messages are reasonable.

Test Plan: Tried to HTTP an SVN repo. Tried to clone a Git repo with SVN, got a good error message.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7562