[ticket/11711] Inform user of unsupported characters while posting

Created PR for develop-ascraeus as discussed with nickvergessen.

Ticket: https://tracker.phpbb.com/browse/PHPBB3-11711
Right now I'd add a method to the db driver to check if 4-byte utf8 characters are supported, optionally if the specified column of a specified table supports 4-byte utf8 characters. Database types like postgresql that usually support this can then just return true by default. Databases like MySQL could return false if no parameters are supplied or check the actual column if database table and column are supplied using a database query.