Make it easier to use backend filters

[SpaceK33z]

Sometimes you want to fetch a store from the backend but with some filters applied.

Our Django-Binder stack has a lot of auto-generated filters. To filter a store on a customer id we can do something like this:

const store = new Store();

store.params = {'.customer': customer.id };
store.fetch();

For a customer filter the backend has defined it looks about the same;

const store = new Store();

store.params = {'.leaves_for_customer': customer.id };
store.fetch();

The syntax for defining this is a bit weird + doesn't use camelCasing. We can perhaps fix it to look like this;

store.addFilter('leavesForCustomer', customer.id);
store.removeFilter('leavesForCustomer');

[SpaceK33z]

Note: think about how to implement filters like '.customer:isnull': true

[SpaceK33z]

Some other notes:

• how to implement '.start_date:range': '2017-01-01,2017-02-01'? I think addFilter('startDate', ['2017-01-01', '2017-02-01']) makes sense.
• add something like store.getFilter('startDate'), because in the frontend you need to show this data.
• how to handle ?deleted=true, since it doesn't have a leading . (+ there can be other custom filters)
• add tests for nested filters like '.customer.location.city': 'Eindhoven'
• for #53 we also need a similar solution (backend sorting)

[SpaceK33z]

Another idea, taking into account the notes above:

store.setParam(Params.FILTER_RANGE, 'startDate', [moment(), moment()]);
store.setParam(Params.FILTER_CUSTOM, 'deleted', true);
store.setParam(Params.SORT_ASC, 'pastOwner.lastName');

Problem with this;

• how to read the value of a param? you can't do store.getParam('pastOwner.lastName') because that can either be an sorthing or a filter or whatevs.