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
Subscribers: avivey, Korvin, epriestley
Maniphest Tasks: T7977, T7220
Differential Revision: https://secure.phabricator.com/D12608
