Fix/replace restler

[adeleKayyyy]

• replaced restler with fetch
• added some functions to reduce duplication

[chrishutchinson]

In general this looks good, a few overall points:

• We'll need to get a solid test suite running to be really confident we haven't broken anything (see my note in Added jest framework #2)
• There's lots of var and other ES5 stuff in this codebase (purely because of how old it is). We could probably consider upgrading this to use let, const, arrow functions, and other good ES6 stuff which has been supported since Node 6 or 8 in most cases. More details of support for specific features: https://node.green/
• It's quite a long file, lots of stuff going on. I wonder if we could modularise it a little? Not sure exactly the best way to do that, it'd be good to get your thoughts on that