Allow properly bound attributes for HTMLBars link-to helper.

Initially I used `readUnwrappedModel`, but that replaces the stream with
a static string (clearly will not be bound at that point). This uses
similar logic as the Handlebars link-to (and `readUnwrappedModel`) to
find the lowest unwrapped model, but preserves things as a stream.
Merge pull request #9611 from rwjblue/htmlbars-link-to-unwrapping

Allow properly bound attributes for HTMLBars link-to helper.