Limit review size and catch bad JSON

[mrclay]

Limits the acceptable size of review entry and formally catches JSONDecodeError instead of litting Django deal with it.

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Additions

• Returns 400 status if request body limit too large
• Returns 400 status if request body is invalid JSON
• Adds unit tests for those

How to test this PR

Run unit tests.

Checklist

• [x] PR has an informative and human-readable title
• [x] Changes are limited to a single goal (no scope creep)
• [x] Code follows the standards laid out in the CFPB development guidelines
• [x] Future todos are captured in comments and/or tickets
• [x] Project documentation has been updated

[mrclay]

@higs4281 @nbucknor Done!