Check and enforce max length for Review text and summary

[Anthony Lenton]

Per https://wiki.ubuntu.com/SoftwareCenter/RatingsAndReviews review text should have at most 5000 characters, and summaries should have at most 80 characters.

The server should enforce this when a new review is submitted and raise an exception if not.

[Michael Vogt (mvo)]

Note to QA, this is not testable with software-center currently because it enfoces those limits too.

[Dave Morley (davmor2)]

Tested. Counter kicks in with -1.... on both the review and the summary.

[Dave Morley (davmor2)]

This is working on reviews.staging and the colour issue from before is resolved too