* Limit number of mentions in a single comment
Ensure that no more than 6 users can be mentioned in a single comment
* Extract max user mentions into constant
* Check mentions but don't process them unnecessarily
* Add error message check to comment spec
* Add MAX_USER_MENTION_LIVE_AT to comments, plus specs
* Bump MAX_USER_MENTIONS to 7 for DEV Top Seven posts
* Add some clarifying comments around MAX_USER_MENTION limit
* Update app/models/comment.rb to use activesupport helper!
Co-authored-by: Michael Kohl <citizen428@dev.to>