[Epic] Notifications

[nick-funk]

What does this PR do?

Creates the basis epic branch from which all Notifications work will merge off of and back into.

These changes will impact:

• [ ] commenters
• [ ] moderators
• [ ] admins
• [ ] developers

What changes to the GraphQL/Database Schema does this PR introduce?

• See the child feature PR's pertaining to Notifications
• Adds the notificationCount query to the Query type

Does this PR introduce any new environment variables or feature flags?

No

If any indexes were added, were they added to INDEXES.md?

See the child feature PR's pertaining to Notifications

How do I test this PR?

See the child feature PR's pertaining to Notifications

Were any tests migrated to React Testing Library?

See the child feature PR's pertaining to Notifications

How do we deploy this PR?

Release details TBD.

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