R1:1eab16c395e3

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…