Fixes document attribute assignment in document batch upload

`.each` was incorrectly being used as if it was `.each_with_index`
Fixes my hasty fix for the document batch POST

Nested forms actually posts numerically indexed hashes, rather than
arrays. Tests updated to reflect this. Original code was working!
Adds number to Document model
Documents batch uploader accepts "number" for Documents
Adds number field to Document edit view
Renders document number in admin table
Merge branch 'documents-add-number' into develop