Fix editor size jumping after resizing

Fix a bug with the auto-expanding rich text editor where it was
shrinking after the user clicked outside the component element.
This was because of a couple of reasons.  First the toolbar was
toggling between shown and hidden states on blur/focus. Second, the
stylehost elements were getting set to the height of the content
and not the height + extra space.

Clean up the way default options and callbacks are handled in
``RichTextEditorMixin.createEditor()`` and add a ``toggleToolbar``
option that, if set to false, will always show the toolbar.

Addresses #844
s/storybase.forms/forms/

Avoid the verbose ``storybase.forms`` in this file because we create
the local variable ``forms`` and initialize it to equal
``storybase.forms`` at the top of the module.
s/storybase.MutexGroupedInputForm/storybase.forms.MutexGroupedInputForm/

For consistency.  Also create a local variable at the top of the
self-executing anonymous function in buider/views.js to make the
following code more compatible with an AMD-based solution if we ever
go that route.