4-byte utf8 chars like emoji are passed around in ruby fine, but
when they are put into mysql queries, strings get truncated at the
first mb4 character. to prevent truncation, strip out mb4
characters in most user-controlled fields like comments, story
descriptions and titles, and messages.
to properly support utf8mb4, mysql server 5.5 is needed, the table
encodings need to be changed to utf8mb4, and the mysql2 gem needs to
be upgraded once it supports utf8mb4:
https://github.com/brianmario/mysql2/issues/249
