Added gui for for choosing newznab (tv) categories

Functional: You can now pull the newznab categories/capabilities from your configured custom newznab provider. You'll get an extra multi select box, where you can select and save the categories which you want to use for automated searches. For example when you want to also search in the TV-Other or Foreign category

Commit desc summary:
Added gui for selecting categories from multiselect box.
Created some helper function in js, for dynamically modifying selects/options

Made results of function for retrieving newznab capabilities more generic. In that now always a valid json is returned with success,tv_categories,error

Added gui elements for retrieving and displaying newznab capabilities

Added backend functions for calling ajax /getNewznabCategories?name=yourNewznabProvider&url=https://newznabprovURL&key=YourApiKey
Returns json.dumps() with TV category capabilities of newznab provider. Is going to be used for new gui element in adding newsnab provider.