Discussed in {T167565}
An example invalid unit being added on wikidata can be seen, https://www.wikidata.org/w/index.php?title=Q420481&diff=prev&oldid=494773425
Units, calendar models and globes should allow the use of items from wikidata.
These would also take the form http://www.wikidata.org/entity/Q\d+
When a non wikidata wikibase is used both a valid wikidata item URI should be allowed, as well as an item on the local wikibase.
The validity of this URI could be checked using the base URI for concept URIs, https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/master/docs/options.wiki#L24
**Acceptance criteria**
[] A simple regex validator is applied for:
[] units
[] calendar models
[] globes
[] On Wikidata.org only wikidata.org items are allowed
[] On a non wikidata.org wikibase both wikidata and local items are allowed