Collection page finalization

[5aola]

Fixed

• num_sounds The old increment/decrement signals were replaced with a single recount (Collection.update_num_sounds()) fired only at post_save and post_delete
• At collection page {% if paginator.count %} replaced with {% if collection.num_sounds > 0 %} which stays consistent across search queries -> fixes search stacking and empty collections.

Added

• Redesigned collection page on the standard two-column layout to match Packs, added:
  • Overall rating stars,
  • Owner info and follow/unfollow button,
  • License summary on the collection page via a new LicenseSummaryMixin shared with Pack
  • Tag cloud built from the collection's sounds (get_collection_tags_bw)
• Visibility setting (Public/Private radio) in the create-collection modal, and a visibility
  indicator on collection cards shown to owners/maintainers
• "Featured" badge and card styling for featured sounds (new $orange color variable)
• Maintainers can now edit featured sounds
• SoundManager.sounds_for_collection(): lightweight accepted-sounds queryset, reused by
  bulk_sounds_for_collection
• Collection.user_is_owner_or_maintainer() centralizing the permission check used by views,
  forms and templatetags

Performance fixes

• License summary resolves the License object once and caches it (benefits packs too)
• display_collection templatetag accepts a Collection instance, avoiding a re-fetch per card

Screenshots about the visual changes:

[ffont]

Thanks a lot, merging this right now!