Replace repository prop with apiUrl prop on wc-gocam-viz component

[pkalita-lbl]

Fixes #25

Currently the wc-gocam-viz component has a repository prop which accepts an enum-like set of strings. Based on the value it builds different URLs to fetch the GO-CAM model. These included an endpoint on api.geneontology.xyz as the default, a couple of Barista endpoints (which we don't want to use at all anymore), and a localhost endpoint which would only be useful for certain local development scenarios.

With these changes the release prop is removed and a new apiUrl prop is added. The default is the new api.geneontology.org endpoint. Since changing the API endpoint should be a fairly rare use case I've also removed the select box from the dev/demo pages. If you needed to point the widget to a local endpoint for development, you'd temporarily change the dev/demo page by adding the appropriate attribute (e.g. api-url='http://localhost/path/to/endpoint/%ID) to the wc-gocam-viz element.

cc: @kltm

[kltm]

Code to remove repository, in favor of apiUrl, with some other minor fixes. Likely does as advertised.

This can be self-merged whenever is convenient for others.