Move duplicate code in feedback plugins to a common trait

Tricky to get all four feedback plugins to work with a common trait, but we got there eventually!

Feedback in the MR :)

Feedback incorporated!

Just one more thing, either a regression or a pre-existing issue with the full text plugin. Should be fixed in the trait if possible so if validation fails when checking for feedback, it's visible on the screen regardless of the plugin used. More info on the MR.

Added minimum character validation in the AJAX callback, hopefully final fixes.

Almost there ;)

Added all errors to feedback area.