R1:2483f6f120a4

Move symbols to be repository-based

Summary: Fixes T7220. Ref T7977. Changes symbols from being bound to an Arcanist project to being bound to a repository.

Test Plan:
- Added symbols and then applied migrations, symbols seemed to be migrated successfully.
- Tested the `/diffusion/symbol/$SYMBOL_NAME` endpoint.
- Tested the `/diffusion/symbol/$SYMBOL_NAME` endpoint with the `?repositories=$REPOSITORY_PHID` parameter.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley…