Add support for Wikibases other than Wikidata

[GreenReaper]

EntityShape currently seems to assume it will only ever be run on Wikidata and contains hardcoded URIs in both Python as well as JS, both in obvious places like _get_property_name and _get_entity_json, but also _strip_schema_comments and _compare_statements.

In practice, Wikidata does not contain all data, and in my use-case I wanted to check our Q3 against our EntitySchema:E1 - which as documented in that schema works with the ShEx2 validator once CORS is whitelisted for that domain with CORS Everywhere.

Seeing this script recommended in Wikidata's shape tutorial, I tried to adapt the JS by changing the harcoded URLs within it and using mw.loader.load() on it. However, this failed. Looking closer, I saw it passes entity and property IDs into the API, not URI/IRIs.

It would be nice if it could work with a different base URI that was passed into it, or potentially across disparate URIs for entity and schema, to aid use of Wikibase by third-parties and federation between these and Wikidata. Failing that, moving hardcoded URIs into centrally-defined constants would probably make it easier to reuse the code if hosted elsewhere (e.g. by @wbstack).