[Maintenance] - Availability Reminders

[staysgt]

Description

Creators of events should be able to re-ping user's who have not confirmed their availability to an event on Finishline.

Acceptance Criteria

• Users receive reminders to fill out their availability for events
• Creators of events are able to ping via a "Remind Unconfirmed" button on the event availability page

Proposed Solution

• Backend endpoint for pinging unconfirmed members
• Add a "Remind Unconfirmed" button on the event availability page that lets the event creator re-send the Slack DM to every invited member who has not yet filled out their availability

Mocks

No response