App: Extract JS method search_category
App: Extract method process_api_results
App: Add method cut_category_prefix

Add method to cut a prefixed-category string into a non-prefixed
'Category:Foo' --> 'Foo'
App: Remove the Category: prefix from autocomplete

We query the API in the Category namespace, so we retrieve
names prefixed with 'Category:', which we do not want.

We remove it my using the map method to apply cut_category_prefix
(introduced in 95ca64f) to each member of the Array.