Implement Gemeente Ede as a source

Implement [Gemeente Ede](https://github.com/openstate/open-cultuur-data/wiki/Gemeente-Ede) as a source

* The CSV file was converted to JSON, then uploaded to the static opencultuurdata endpoint
* We now have a station JSON importer as well (It's different from the dump importer)
* There's a slight issue with get_all_text() which I wasn't sure how to solve. I've added the call in the item transformer to make sure the tests pass
This implies that when importing a static JSON file, it always contains the items as an Array in the root of the document. Will this always be the case? Is so, we choose a fixed data format and should document this in the docstring of the ``StaticJSONExtractor``. If not, we should make the selection of items somehow configurable.