First pass at auto-expanding text editor

The WYSIHTML5 editor makes it pretty hard to implement this. One of
the major challenges is that the editor copies the styles, including
height of the base textarea element, to initialize the rich text
iframe.  It then uses the copies of these styles to reset the iframe
styles whenever the editor loses/gains focus.  So, I had to read
through the source to figure out how to make sure the new height
we set isn't clobbered.

Updated ``RichTextEditorMixin.createEditor()`` to take an optional
``options`` argument that are passed on as options to
``wysihtml5.Editor()``.

Still to do:

* Make the auto-resize an option for all editors
* Get it to size down when the text decreases. This is easier
  said than done because there isn't a nice event to hook into
  and we'll probably have to listen to keypress events.

Addresses #844
Move editor growth initialization into createEditor()

Move the initializaiton of the event listeners on the
``wysihtml5.Editor`` instance to the
``RichTextEditorMixin.createEditor()`` method so that this
functionality can be used in places other than just text asset
bodies.

Also removed a hack that disabled the default event handlers from
the editor instance because it no longer seemed to be needed and
would have broken the auto-grow functionality.

Addresses #844
Add missing semicolon
Hide body field label when editing text assets

Addresses #844
Add box sizing styles to wysihtml5 editor

The editor JavaScript copies some styles from the textarea, but
not the box-sizing styles.  This causes the editor iframe to be
too wide in the side-by-side layout.  Adding styles that explicitely
target the editor iframe (``.wysihtml5-sandbox``) fixes this.

Addresses #844
Removed percentage height from asset wrapper element

The wrapper div around the asset element had a percentage height
of 94% specified. In the side-by-side layout, when one of the
assets is a text asset, it caused the wrapper for the adjacent asset
to grow alongside the text asset as the editor auto-expanded. I can't
see any reason for the percentage height, and can't remember why we had
it, so I just removed this style.

Addresses #844
Merge branch '844-expanding_text_editor' into develop