Truncate block reason for whole multibyte characters

This applies to Special:BlockList, or to the block message shown to the
user, or API results. Log is not effected, because it already truncates
on its own.

Change-Id: I0a8bae5f43fd1604034d65e96c9461a9f83c0b0a
Merge "Truncate block reason for whole multibyte characters"