bake: Commonize worker pool code between html and asset baking. The `workerpool` package now defines a generic-ish worker pool. It's similar to the Python framework pool but with a simpler use-case (only one way to queue jobs) and support for workers to send a final "report" to the master process, which we use to get timing information here. The rest of the changes basically remove a whole bunch of duplicated code that's not needed anymore.

path_mode: absolute
files:
  - bower_components/jquery/dist/jquery.js
  - bower_components/bootstrap/js/transition.js
  - bower_components/bootstrap/js/collapse.js
  - bower_components/bootstrap/js/scrollspy.js