Add Artist+Label smart filters using custom tags

1. Load available tags when open smart playlist modal (for display purposes).

2. Add a new button to the bottom of the smart playlist modal, called 'Add custom tags', to allow the user to add new tags from the smart playlist modal. If new tag added, re-run the filters to generate matching tracks and update the tags array (as per #1 above).

3. Add 'Artist' and 'Label' rows, with tags as the options.

This is so users can create tags to use as custom filters for artist and label!