Adding changes to prompt meme model
3354 nonfunctioning navbuttons for noncanonical tags
change css cleaner to allow vendor prefixes; remove broken css comment-parsing logic
Merge pull request #1161 from Talvalin/issue_3397_PM_more_prompts_required_allowed

Adding changes to prompt meme model
Merge pull request #1058 from tuff-ghost/issue_2834

2834: CSS Sanitizer inconsistent on allowed browser-specific prefixes
Merge pull request #973 from sarken/issue_3354

3354 remove nonfunctioning buttons for noncanonical tags