fixes view state and err msg
U4-4045 The "Empty" partial view snippet should have @inherits
fixes the protect page logic
Fixes: U4-4010 Member and user session gets mixed - fixes how we detect a back office request + unit tests and added new IsBackOffice attribute
Merge remote-tracking branch 'origin/6.2.0' into 7.0.2

Conflicts:
	src/Umbraco.Web.UI/Umbraco/PartialViews/Templates/EmptyTemplate.cshtml
	src/Umbraco.Web.UI/umbraco/dialogs/protectPage.aspx
	src/Umbraco.Web/Umbraco.Web.csproj
	src/Umbraco.Web/UmbracoModule.cs
	src/Umbraco.Web/WebApi/UmbracoAuthorizedApiController.cs
ensures webapi plugincontrollers are routed to /backoffice if [IsBackOffice] is specified
Merge remote-tracking branch 'origin/6.2.0' into 7.0.2
ensures auth controller has IsBackOffice specified