Fix for TIM-474. Disable submit buttons during transaction.

@wedaly @ormsbee @jrbl 

See the JIRA ticket for more details on why this is important, but essentially, we don't want people clicking more and more if there is a bit of latency. 

https://edx-wiki.atlassian.net/browse/TIM-474

Essentially:
Disabling a button when a peer or self assessment is submitted.
Re-enable the button when there is an error.

On success, the section is re-rendered. Additional logic to re-enable the button would be a moot point, since it would get re-rendered from the template.