As an editor, I want to refer to existing EntitySchemas in statements on Items, Lexemes etc. in order to make it easier to find and classify EnititySchemas. We need a new datatype to link to EntitySchemas in statements.
**Example:**
Human (Q5) is governed by the EntitySchema Human (E10).
We would like to link these via statements to show the link between the Item and the EntitySchema.
**BDD**
GIVEN an Item
AND an EntitySchema that governs that class
THEN a new statement can be added that links the Item and the EntitySchema
**Acceptance criteria:**
[] a new datatype exists that allows linking to EntitySchema
[] Only existing EntitySchemas can be selected as values
[] Link text should be the label of the EntitySchema
[] The value of that link should be exported in RDF as a URI
[] "What Links Here" on an EntitySchema should list all entities (Items, Lexemes etc.) linking to it through the new datatype
**Notes**:
* The value input of a statement should have the ability to use EntitySchema ID as well as the label of the EntitySchema. If the EntitySchema label is unable to be used in this instance the EntitySchema ID is acceptable.
* This property proposal has been approved and is awaiting this datatype: https://www.wikidata.org/wiki/Wikidata:Property_proposal/Shape_Expression_for_class
* Datatypes overview and definitions: https://doc.wikimedia.org/Wikibase/master/php/docs_topics_datatypes.html