Fix/adam/log answers with post

Addresses part of LMS-467, where student information was being sent to the tracking log via GET query parameters, which was causing nginx to throw 400 errors when the urls got too long. This information is now sent via post. I also added a limit to the size of the JSON strings that are sent to the logs.

Link to LMS-467 in JIRA: https://edx-wiki.atlassian.net/browse/LMS-467

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