Project Browser makes a pointless API request for categories

Problem/Motivation

Source plugins have a getFilterDefinitions() method, the base implementation of which calls $this->getCategories() to produce a list of categories as one of the filters.

The Svelte code also makes an AJAX request to the backend, which...calls getCategories() on the source plugin. Since the categories are available up front when the Svelte app loads, this request is completely pointless, makes the code more confusing, and necessitates an extra route and method of EnabledSourceHandler which is not ultimately needed.

Proposed resolution

Remove EnabledSourceHandler::getCategories() entirely, and the route which calls it. Then refactor the MultipleChoiceFilter component in the Svelte code so that it uses a set of categories passed to it from drupalSettings. This will allow us to remove three of the most perplexing writables in the Svelte app (categoryCheckedTrack, moduleCategoryFilter, and moduleCategoryVocabularies).

As part of this issue, we'll also need to make sure that categories are internally cached by the source plugins (they are currently cached in a non-volatile way by EnabledSourceHandler, but I think it's probably fine to have them in a regular volatile cache).