Adding toggle for who to notify on Slack

[EdgarButtner]

Changes

I added a frontend toggle to the create and edit event form on the calendar page. You can now choose between notifying individuals and channels. To save who the event wants to notify I added a field to the Event model in schema.prisma. This allows us to correctly save and render the toggle in the frontend when editing an event and seemed to me like the best way to do this.

Notes

I think modifying schema.prisma was the right way to do this but if not please let me know and I'll change it ASAP.

Test Cases

• Added test @user is set as default
• Added test making sure @channel persists across page refreshes and is stored correctly

Screenshots

See Toggle to the right of the notiification section

To Do

Make Tests

• Tests for who we choose to notify

Checklist

• All commits are tagged with the ticket number
• No linting errors / newline at end of file warnings
• All code follows repository-configured prettier formatting
• No merge conflicts
• All checks passing
• Screenshots of UI changes (see Screenshots section)
• Remove any non-applicable sections of this template
• Assign the PR to yourself
• No yarn.lock changes (unless dependencies have changed)
• Request reviewers & ping on Slack
• PR is linked to the ticket (fill in the closes line below)

Closes #4093

[staysgt]

Looks really good, just two small comments, one above and: in calendar.routes.ts, in the create/edit event routes, validate the mention parameter

[staysgt]

small, but these should be all caps