Add top bar and offset model within interactive

Initial work for starting to make interactive look
like Upstatement's work. Add blue background, white
top-bar, and update layout to pad inner part of interactive 
to leave a little room between borders and model (and all 
other components with top/left of 0).

[#44616109]
Add radial gradient to background and curved corners

[#44616109]
Rm iframe border

Now that we have a clear frame, we should view it the 
way it would be embedded in a webpage. iframe resize 
handle is still visible and functional.
Create bottom bar for frame; move credits to it

Arrange components by reducing "availableHeight" by
size of bottom bar.

[#44616109]
Make about link always visible, show credits if no About

To keep standardized layout, we should always have an
About link, which will show the CC credits dialog if
there is no other About text.

[#44616109]