Allow nulls in `projects.host_connection_{id,server_id}`

The server version on stable won't be able to fill values for those
columns when we deploy the migration to preview.

With this commit we're also dropping the unused `worktree_extensions`
and `project_activity_periods` tables. The last version of the server
on stable (0.2.6) doesn't contain any code that accesses those tables.
86e5ae1f2eAntonio Scandurra committed on 12/15/2022, 10:30:51 AMparentaadd7f2
5 files changedLine totals unavailable