Change availability API format to list of timeslots

**What needs refactoring?**
The format of availability painting API data.

**Why?**
Currently, the data passed into the availability submission endpoint is a grid of booleans. This is a burden on the frontend to convert the "available timeslots" that are represented by a list of ISO strings to a grid.

This should be changed on both the frontend and backend, both for efficiency and reducing error.