Allow for absolute URLs as well as virtual in ResolveUrl to allow for FileSystemProviders that use CDNs
Obsoletes UmbracoFile with new UmbracoMediaFile in the Core assembly and wraps the legacy class to use the new one. Improves performance of UmbracoMediaFile by ensuring any IO is lazy loaded and then saved to a local property so it doesn't need to re-read the file for things like length/size.

Conflicts:
	src/Umbraco.Core/Umbraco.Core.csproj
2nd attempt: U4-2514 Checkboxlist prevalues in packages broken
Merge branch '6.1.2' of https://github.com/stevetemple/Umbraco-CMS into stevetemple-6.1.3
Fixes U4-2187 - Enable support for MySQL on Linux
Unit test for: U4-2514 Checkboxlist prevalues in packages broken
Fixed runtime cache provider issues & unit tests.
Fixed Template model class to not have it's own NodePath property which was always null since it wasn't mapped (and there was no point in it).
Fixes up validation styling and the validation summary close button display.
Fixes Allow At Root styling
Fixed editContent.aspx to let the business logic handle the property validation when trying to publish and show the correct status message result (if it was not publishable). Adds the invalid properties collection to the PublishStatus so those can be shown too.
Fixes: U4-2535 The actions/publish.aspx page does not publish correctly and show the correct publish result
Merge remote-tracking branch 'origin/6.1.3' into 7.0.0

Conflicts:
	src/Umbraco.Core/Umbraco.Core.csproj
	src/Umbraco.Tests/Services/Importing/ImportResources.Designer.cs