Move repository URIs to a dedicated index

Summary:
Ref T4705 (there are also some other adjacent related tasks dealing with URIs).

Currently, we issue a "get repositories matching URIs: ..." query by loading every possible repository and then checking their URIs in PHP.

Instead, put URIs in a separate table. I plan for each repository to potentially have multiple URIs soon, so this prepares for that.

Test Plan:

• Ran migrations.
• Looked at index table, made sure it appeared sensible.
• Ran some queries by uri to find repositories, found the repositories I expected.
• Updated the remote URI of a repository, saw queries / index update appropriately.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4705

Differential Revision: https://secure.phabricator.com/D15005