Make use of new data model constructors all over the UI now

* replaced wb.entities with wb.fetchedEntities, holding wb.FetchedContent now instead of wb.Entity
  instances. Includes some tests.
* wb.entity is a wb.Entity object representing the site's entity now.
* for unserializing JSON in entityViewInit we now use the wikibase.serialization module.
* wb.entityview is expecting a wb.Entity object instead of a loose data structure now.
* removed/changed some todos

Change-Id: I40bcc945610fe6f32e89fc12ea3b0def3126317a
Merge "Make use of new data model constructors all over the UI now"