Implements new starterkit rest call

To overcome the obstacle of the new starterkit only being compatible with 7.2.0.
In this version the starterkits are sorted by sort order in the Our repository. If you install using the "express" method (not customized) then the first starterkit returned will be used.
Adding things to `InstallHelper` felt a bit dirty, maybe there's a better way..
If Our is down, it will fail with a log message and no starter kit will be installed.
Please always refrain from using singletons when they are available on the actually class you are using. The InstallApiController has a property called UmbracoContext, use that. Singletons are basically bad practice, do not use them unless it's a last resort. Everything should be created with all of it's dependencies (i.e. constructor injection). Moving forward ASP.Net 5 will force this on you. Nearly all of our base classes have ApplicationContext and UmbracoContext as properties which can be injected into their ctor's... this allows for better unit testing and Ioc