Notifications: show "load new" button at top of notifications feed when available

[nick-funk]

What does this PR do?

Shows a Load new notifications button at the top of the notifications feeds when new notifications come in while the tab/floating-tray/mobile-tray is already open.

These changes will impact:

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

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

None

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

No

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

N/A

How do I test this PR?

• post a comment as a user
• open the notifications tab/trays as a commenter
• with a different user, reply to the original commenter
• see that Load New appears at top of the notifications tab/tray for the first user
• click it
• see that the feed loads in the new notifications

Were any tests migrated to React Testing Library?

No

How do we deploy this PR?

Merge into epic branch.