U4 5512: Load Angular locale files dynamically

This pull request makes Umbraco load the correct locale file for Angular.js for the logged-in user's locale. Angular does not support changing the locale, so I ad making use of this library: http://lgalfaso.github.io/angular-dynamic-locale/ (MIT license)

This is my first foray into the Angular parts of Umbraco 7. I think I have done everything correctly, but you never know...