assets: requirejs exclude option

* Adds support for exclusion of files in RequireJS builds, to prevent
  files from being included multiple times. (closes #2411)

* Enables more aggressive compression for UglifyJS.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
base: requirejs improvements

* Several improvements for better requirejs compliance.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>