Admin check: fix MySQL UTF-8 collation check

MySQL 5.5.3 introduced the 'utf8mb4' character which allows storage of
up to 4-byte unicode characters (utf8 only allows 3 bytes) [1].

As a consequence, the current check for collation beginning with 'utf8_'
returns false if the collation is utf8mb4.

This adds a new check_api check_is_collation_utf8() function to ensure
consistent checking of collation, and removes the underscore from the
comparison string.

Fixes 0020426

[1] https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html