Split off itemview and propertyview from entityview

- Made entityview abstract
- Moved statement handling to itemview and propertyview
- Moved site link handling to itemview
- itemview and propertyview can be created without any pre-rendered static HTML now

Needs https://github.com/wmde/WikidataBrowserTests/pull/32 to not break tests!

Change-Id: Ia417c45af10c881a5edd4d2ef82649492922be05
Merge "Split off itemview and propertyview from entityview"