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
(bug 44636/bug 44689) Deleted properties handling by the JavaScript UI

The UI will no longer fail if there are Snaks using properties which have been deleted.
Also allows to still edit (as far as possible) and remove Claims and References with
Snaks using delete Properties.

Change-Id: Id619259d5b29ba9b1a4d89437b21fdc362d2946b