Added duration limit to songs.
Handle missing config values with warning and default values.
Song sources can set "displaytext" on rejection errors now.
Config value checker now warns about missing values in missing objects.
Added ConcurrencyLimiter for limiting concurrent jobs.
Limit number of song transcoding jobs happening at once.
Searching makes independant request for each section.
Don't act on escalate button press if song is already next.
Fixes relative date in activity feed showing "in a few seconds".

Resolves issue #27.
Possibly fix issue #24, where clicking search input closes search.
Incremented version to 1.3.0-alpha.
Merge branch 'dev' at 1.3.0-alpha.

Major fixes:
  #20: Searching uses independent request for each search result section.
  #24: Clicking search input no longer closes search on some browsers.

Minor fixes:
  Escalate button no longer does anything if song is already next.
  #27: Date in activity list can no longer show "in a few seconds".

Features:
  Can limit number of song transcoding jobs happening at once.
  Can specify duration limit for songs to add.
  Some song-adding errors can be shown to user if "displaytext" is set.
  Server gracefully handles missing config values.