Initial extension work

Needs refactor to remove code duplicated from components.
Temp fix to restrict bower search
Temp fix for package version
- Loading overlay added for app
- Fix for #380 - using assetmanger for asset serve
Merge pull request #384 from adaptlearning/loading-overlay

Loading overlay added
Remove redundant model
Add description to extension/components
Merge branch 'extensions' into develop
The beginning stages of the extensions UI

Work in progress
Merge branch 'develop' into enable-extensions
On-going work to 'Manage extensions' UI

Awaiting API end-points