Consolidate search results and artist pages

Search results no longer separate results by spotify and local tracks.
Mopidy has support for several plugins and it no longer makes sense to
only support spotify and local backends. To that end, the local tracks
tab has been removed from search pages and artist pages. These results
are now displayed as just tracks.
Merge pull request #13 from connrs/feature-consolidate-search-results

Consolidate search results and artist pages