Stop not updating form controls if they're focused

On Blaze, we copied functionality we had on Spark: Don't update
form controls (INPUT, TEXTAREA) if they are focused and their underlying
value reactively updates. This was never meant to be the eventual
solution -- we'd eventually have a way to define strategies for two-way
data binding. Maybe you'd be able to define a callback that notifies
the app when a change happens to a field that hasn't been saved yet.

Moreover, not only is the feature incomplete, but with Blaze it works
much more poorly than in Spark. Due to fine-grained updates, users
his this more frequently and don't seem to like the behavior
(in Spark you would only hit this behavior if you set up your
preserve rules exactly right, which many users did not do).

So, we're just ripping out this functionality. Now if a field gets
edited by some other user while you're focused it will just lose its
value. Focus will remain.

Fixes #1965