Reintroduce `Views` support

[JeremyCaney]

Previously, the Topic Editor had support for views. This was (apparently) removed due to lack of full support. This should be reintroduced in the ASP.NET Core version.

• Add a TopicList of LookupListItems called Views to ContentTypeDescriptor
• Add a TopicLookup bound to Views to ContentTypes (so it'll be available on all topics)
• Add a special {ContentType} token to TopicLookup to enable the above reference

This way, each content type can have a list of views associated with it, and the author of each topic implementing that content type can select one of those views from a dropdown list.