Organise API endpoint utilities

Just re-organising the functions into separate files for now. I've split the IDs server endpoints into their own file too and added an ids.js config file.

Usually, the functions in an api folder return promises (e.g. return axios.get(...)). At the moment, axios is called in action creators. Since we're gonna move to Rest Hooks and remove a lot of these actions, I didn't bother changing anything on that front.