Add user flags feature, template bug fixes, top rated template, JSON for Subsonic API

This is a merge request with few changes. Major changes are:

  * Add a new "Top Rated" link in home menu to view the top rated songs, albums, artists.
  * Fix rating on stats pages and cell alignment when rating disabled.
  * Add a new 'user flags' to flag songs, albums and artists per user. This was a missing feature ; administrative flags doesn't have this purpose, and flag != rating. For comparison, User Flags is to Ampache what Stars is to Subsonic. Feature disabled by default and has to be activated in config file. Once activated, it add a new Flag icon column near rating stars.
  * Add JSON format support in addition to XML format for Subsonic API. Fix several Subsonic clients compatibility (MiniSub, UltraSonic ...)

This add a new user_flag table, with update identifier set to update_360017, after pending pull request #77. Please change the update identifier if you want to merge this pull request but not #77.