Saved search improvements

Created by: sqs

Placeholder tracking issue for saved search improvements in 3.3

• Saved searches not appearing on homepage #1364 (closed)
• Move saved searches from settings to a dedicated table in the database (stretch) (Not going to be completed, will be shipped in 3.4)
  • Can't delete or create multiple saved searches in one session. https://github.com/sourcegraph/sourcegraph/issues/3208 (stretch)
  • Make saved searches available through our GraphQL API (stretch)
• Re-implement notifyOrganizations and notifyUsers (removed in https://github.com/sourcegraph/enterprise/pull/9962, issue https://github.com/sourcegraph/enterprise/issues/9961) (stretch) (not going to be completed)
• Configure Slack webhook URL link is broken #3102 (closed)
• Add tests to react components (stretch)