[Design] Rework `ViewDataDictionary` constructors to ensure `ModelMetadata` is ne...

...ver `null`

- `ViewDataDictionary<TModel>` constructors now pass `typeof(TModel)` to base constructors
- remove `ViewDataDictionary<TModel>.ModelMetadata` override
- add more doc comments

nits:
- change `RazorPageActivator` to pass current `ModelStateDictionary`
- change 2 other `ViewDataDictionary` copy constructor calls to avoid
  uselessly copying `Model` and related information from source
- make a few `ViewDataDictionary` properties get-only
```
return (ViewDataDictionary)_typeActivator.CreateInstance(
    context.HttpContext.RequestServices,
    activationInfo.ViewDataDictionaryType,
    context.ViewData);
```
All you really need here is to call the generic constructor no? This could be done a few different ways, let's not jump to a conclusion.