Actually build and adopt an attendance register with API

## I want to do this:

I want to build (or redevelop a good example of) this Final Project and actually adopt it into use.

https://docs.google.com/document/d/1BQd_AA4qz-N1fv83yeyh72QdhSnAzdn6GtwjC6tMwDQ/edit

Loads of people really want this project! It has been built a couple of times but not yet taken forward in a way that can really be useful.

## Here’s why I want to do it:

- We know that attendance is highly correlated with success on our course
- In some schools, we are inconsistent with recording attendance, so we miss trainees who are falling behind or having personal difficulties
- It's laborious for volunteers to manually update attendance in a spreadsheet when we are a community of developers so should not be burdened by this kind of data entry

## Here’s how it serves our goals:
- This would be a decent portfolio piece for our grads
- This would free up our time and be simpler to handle
- This would be easier to make available to a new tracker
- This would be more reliable and therefore more useful to everyone

## This is how much time I can put towards this change:
### already done
- I have written this Final Project briefing
- I have explained the idea to various CYFers who also really want this
### can do
- I'm happy to throw on a nice, accessible and simple front end, so don't get bogged down in that stuff - focus on the function
- I can offer up to 6 121 sessions for development and 3 for testing

## This is the help I need from others to get this done (if any):

- a team of two developers to build this
- a liaison from @CodeYourFuture/cyf-next to make sure the API makes sense
- a tech volunteer to code review
- a product owner to drive adoption