added comment model
remove the problem_comment and solution comment models
removed problem and solution comments from database
removed the comment concern and included votabale in comments
git change problem and solution model
comments is working
removed the last of the problem comments and solutions comments. fixed #156
Merge pull request #157 from codeflash/feature/polymorphic_comments

Feature/polymorphic comments
Merge branch 'dev' into feature/pygments