(RFC) TIM-346: Server-and-client-side size validation

Pushing for comment and feedback on some parts that I'm unsure about:
* @Lyla-Fischer what should be the input size cap? Should there be one, or potentially different ones for each of submissions, assessments, and assessmentfeedback? Does the cap need to be configurable in any meaningful way, or is some site global sufficient?
* @talbs This needs some styling love. In particular, the divs I added to oa_response.html and a few other places to tell the user why they're getting this error message. Right now it's defaulting to gray text on a red background; it's pretty bad. I can just make this something that seems ok if you won't have time to mess with it for a while.
* @wedaly Can you suggest how to write spec tests for input size validation? I don't want to actually make a submission of a given size and wait for it to fail, as I do in the python tests, do I?

Things I still dislike:
* Error messages for AssessmentFeedback going to console; I wasn't able to get the error message div I created to toggle with toggleError
* Sizes are configured in too many places, when they should come in via settings or something

@stephensanchez fyi this is my current state on 346 at pacific bedtime.