Implement framed embedding support.

This is a first step in integration with learning management systems like EdX or CourseBuilder.

This commit supports a special user "frame.pencilcode.net":
loading code into this user will disable all the normal top UI
as well as saving functionality; and it will just show the
two-pane view with an editor and run panel, bare.

This is done (mainly) by just serving up framed.html instead
of editor.html: framed.html has a number of elements removed
or set to zero-size.

Embedding is supported by a client script pencilcodeembed.js.
This script knows how to create an embedded Pencil Code,
and it knows how to load code into it.