[Timetabler] Refactor client calendar saving to send all at once

[CammyFentreux]

I haven't looked close enough to this to know if the MySQL would work, but I think theoretically it could be done all at once saveAvailability and saveAvailabilities in UserClient.js

[joe-herbert]

This can be done just using the values syntax (https://stackoverflow.com/a/6889087/4437968), I'll do it over the weekend unless you want to give it a go yourself.