Add support for utf8mb4 encoding to make possible use (direct storing) of the unicode characters outside of the Basic multilingual plane range (such as Emoji).

Just for the info, some BB utf8mb4 use stats (which I could get):

Invision Community - utf8mb4
XenForo - utf8mb4
Flarum - utf8mb4
Vanilla Forums - utf8mb4
vBulletin - utf8 (default), utf8mb4 (supported)

myBB - utf8 (default), utf8mb4 (supported)
Simple Machines - utf8
PunBB - utf8
phpBB - utf8