Add support for the data type 'external identifier' for statements and qualifiers on commons
A external identifier data type is essentially a string with constraints - on wikidata it's entered as a string, but it's validated against the property's constraints (it seems - https://www.wikidata.org/wiki/Property:P269 under constraints)
Users need to be able to add an external id for any property that takes external identifiers as a value, in the File page and in UploadWizard
##Examples
A quantity can be population, maximum capacity, floors above ground, and numeric value, or others (see full list of Wikidata properties that use the quantity data type).
##User story
As a Commons editor, I want to add a quantity to a media file’s metadata, so that anyone using that file can know a particular kind of quantity pertaining to that file, including the unit of that quantity (optional).
##Acceptance criteria:
[] can select a property with data type 'external identifier' and enter a value for it, on File page and UploadWizard
[] on File page
[] in UploadWizard
[] incorrect values return an error
[] add values other than whole and decimal numbers