ORA-61: Answer too long error message

[ORA-61](https://openedx.atlassian.net/browse/ORA-61)

It turns out that we were restricting *character* length on the client side (using the textarea's `maxlength` attribute), but checking JSON-encoded *byte* length on the server side (in the submissions API).  Depending on which characters were being encoded, a user could exceed the byte length limit without exceeding the character limit (for example, double-escaped newline chars, non-ASCII unicode, and JSON keys).

This PR addresses the issue in two ways:

1) It suppresses the "field errors" returned by the submissions API, which (a) weren't localized and (b) were a scary `repr` or a dictionary, which users should never have to see.

2) It intercepts "answer too long" request errors from the submissions API and displays the more specific error message: "Your answer is too long."  This is somewhat vague (it doesn't tell the user *how* long), but I think it's clearer than saying something more specific like "JSON encoded byte length > 100K", which the user doesn't have any control over.

There's a related PR in edx-submissions: https://github.com/edx/edx-submissions/pull/7
Once that is reviewed, merged, and released, I'll update the requirements to point to the new version in PyPi.

@srpearce Please review the messaging changes (all in `SUBMIT_ERRORS` on line 29 of `submission_mixin.py`)

@stephensanchez and @gradyward please review the code changes.