Support Razor views in Shell Modules

Description

Shell modules have traditionally only supported WebForm views per default. As Razor views are now the default choice, we should make it easier to use Razor views by allowing Module developers to select which View Engine to use for their module.

To select a view engine, change the 'viewEngine' attribute in the module.config to 'Razor'. Standard Razor configuration in web.config is still required.

In addition, it should be possible to opt out of any view engine registration by setting the module configuration to 'None'.