fix(build): make custom builds on demo site work

Use approach from Twitter Bootstrap. Most of the code is from their customizer.

- use modules in mappings file
- generate concatenated JS
- generate tpl and non-tpl file with banner
- generate zip file for custom builds
- add browser compatibility message
- speed up page by deferring loading of files
- only load the files when needed. Cache them after they're loaded.

Fixes #2960
Fixes #2847
Fixes #2625
Fixes #2489
Fixes #2357
Fixes #2176
Closes #2892