Category addition & list for iOS app

Data model:
* adds 'categories' string prop to FileUpload records (pipe-separated list)
* adds 'Category' records with recently-used cats
* shows recently-used cats

UI:
* tweaks 'License' section of details view to show a stub license selector
* adds 'Categories' section to details view which lists selected cats, breaks out to a search addition dialog
* categories recently used show up in list before searching