Migrate saved searches from settings to dedicated Postgres table: DB changes and migration
Created by: attfarhan
Split out from https://github.com/sourcegraph/sourcegraph/pull/3512.
This commit includes:
- the migration to create the new
saved_searches
table, - the migration logic to existing saved queries from settings into the new table, and
- additions to
pkg/db
to manage the new table.
Test plan: