Shortened keys from string(256) to 128 (utf-8 problem)

string(256) is too long (at least on a utf-8 mariadb) for a key.
I picked 128 just because it's a nice number. Not sure what the actual *character* limit (767 :question: *bytes* or something).
I hope it's not too long for existing keys on existing installations, and that it's enough *practically*.