Add caption input fields

This will add a caption field that looks pretty much the same
as the existing description field, but will send its data to
wikibase (if enabled in settings)
If that's not enabled, the captions field just won't be shown.

Submitting uploads have gotten a bit more complex:

• first we have to submit the file & wikitext
• then query page_props for the entity id
• then submit the captions

Bug: T188502
Change-Id: Ia4c58f3c9f82fbcb1d52a949acb4e148cc798cde