Organize external frontend dependencies in static/third_party.

... and update jQuery.
Update bootstrap to 3.1.1.
Fix styles after Bootstrap upgrade.
Overhaul the chords presentation control gadget.
[songs] Extract old url redirects to obsolete.
[songs] Clean up some redundant code.
Replace remaining icons with glyphs and remove unused images.
Remove versioned carousel images as it is now dynamic.
[songs] Refactor logic for lyrics parsing/rendering.