backport r12453 AbstractAdapter change to 2.4-stable (#15756)
Don't load redcarpet with JRuby.
Adds experimental support for Markdown formatting with redcarpet (#15520).
Rescue syntax highlighter exceptions.
Adds an option for formatter display name.
tagged version 2.4.2
Merged r12447 for 2.4.2 release.
Updates for 2.4.2 release.
Bulgarian translation updated by Ivan Cenov (#15755)
Fixed slow SELECT ... FOR UPDATE query when deleting an issue.
