Upgrade dependencies and other improvements

[rkashapov]

Thank you for the great UI for mopidy!

I've upgraded the dependencies and implemented some improvements:

• Load playlist info appeared in Browse page. Previously it displayed "0 tracks" for any playlist there
• Load subdirectories covers on the Browse page. Previously album art wasn't loaded for directory ref types.
• Display extension-defined covers for top-level directories on the Browse page.
• Don't display directories as tiles on the Browse page.
• Pass title argument for subdirectory names.
• Removed static directory content. It is populated while building frontend.

Sorry for the big diff