Allow extensions to add Angular modules

This change allows extension plugins to make use of AngularJS without
changing horizon template or javascript files.
Horizon bootstraps AngularJS once when a page loads with a list
of modules. An extension can register a list of modules and a list
of javascript files that contain these modules.

Implements blueprint: plugins-angular
Change-Id: I4a76363dd4a631cc0d2fc2c902c2f5cac0f073f2
Merge "Allow extensions to add Angular modules"