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

Subscribers: avivey, Korvin, epriestley

Maniphest Tasks: T7977, T7220

Differential Revision: https://secure.phabricator.com/D12608