Implement Express Check-In

[farisashai]

New feature to be implemented for non-logged in users to check-in to their first ACM event with an email only and receive their points retroactively when their account is fully created.

Don't have a design yet but it should be pretty quick with the existing Design System for the logged out user pages.
Design here

But please add a little blurb explaining how express check-in works to the page.

On hold until back-end API support is implemented acmucsd/membership-portal#344
Documentation for backend route is here
View the email that users receive to get an idea of the whole experience acmucsd/membership-portal#344 (comment)

• Implement express check-in page itself
• Add blurb explaining how express check-in works
• Write backend API request logic
• Link to the page from login and registration pages