Refactor requests handling

[SpaceK33z]

Currently requests in a model or store are done via src/request.js. This is kind of a global module. It doesn't offer enough flexibility and makes unit testing harder because it sets variables like baseUrl and csrfToken in the global module.

We could refactor this to a Request class. The user can initialize a Request class in it's own code (only needs to be done once), like this:

import { Request as BRequest } from 'mobx-binder';

class Request extends BRequest {
  baseUrl = 'http://localhost:8080/api/';
  csrfToken = 'xxx';

  get(...args) {
    // do custom stuff
    super().get(..args);
  }
}

If you want to make requests with a model or store, you need to pass in an initialized Request class. Otherwise it will give a clear error message that you need to pass such a class.

[SpaceK33z]

This is nearly finished now, the only thing remaining is adding a clear error message when the api prop is not defined on a model.