TIM-346: Server-and-client-side size validation

Inputs in any user-editable text box that are too large will be rejected
without an AJAX call.

Inputs delivered via Python to the API which are too large will be
rejected with an approriate API error.

TODO:
 * @talbs .message__content is styled very badly everywhere
 * Sizes are way too big, currently defaulting to 1MB for API and 256KB
   for js
 * Sizes are configured in too many places, when they should come in via
   settings or something
Updating size limitations to be more reasonable

Javascript validation keeps inputs < 64KB, the python API allows up to
100KB.
revising visual styling (color/alignment) of inline messages
Updating error text for overlong inputs

Updating error text for overlong inputs from "Text is too long." to
"Response text is too large. Please reduce the size of your response and
try to submit again."
Merge branch 'jrbl/tim-346-check-input-sizes' of https://github.com/edx/edx-ora2 into jrbl/tim-346-check-input-sizes
Tim-346: Addressing PR feedback
Merge pull request #211 from edx/jrbl/tim-346-check-input-sizes

TIM-346: Server-and-client-side size validation