Fixed queue not restricted to container height on firefox.

Also fixed bottom of queue being cut off in firefox when it has to
scroll.
Fixed search input shifted up on firefox.
Replaced 768px in less with variable.
Fixed heights of columns and previews box in single-column mode.
Merge branch 'firefox-css-fix', fixing css for firefox.
Added selected/hover style to search results.
Created SongSourceMap model and populated table.
Fixed search results not covering queue placeholder.

Also changed "Uploads" panel to read "Adding Songs".
Enqueueing of searched songs works.
Correct song is added from search results now.
External song sources are now loaded.
Refactored song source initialization with promises.
Refactored song source search function with promises.
Completely remove async library.
Refactored song sources themselves to return promises.
Set maxlength on search input field.
Implemented external song fetching.

Handle metadata separately.
Added a thumbnail_url in addition to image_url.
Album art is scaled properly. Search results show thumbnail_url.
Merge branch 'enqueue-search-results', allowing adding of song results.

Created SongSourceMap model to map search results to song models.

Song search results can be clicked on, and this either enqueues an existing
song, or fetches it from the source.

Refactored song sources to use promises, and removed async library.

* enqueue-search-results:
  Album art is scaled properly. Search results show thumbnail_url.
  Implemented external song fetching.
  Set maxlength on search input field.
  Refactored song sources themselves to return promises.
  Completely remove async library.
  Refactored song source search function with promises.
  Refactored song source initialization with promises.
  External song sources are now loaded.
  Correct song is added from search results now.
  Enqueueing of searched songs works.
  Fixed search results not covering queue placeholder.
  Created SongSourceMap model and populated table.
  Added selected/hover style to search results.
Incremented version to 1.0.0.