Remove MySQL utf8 option in favor of utf8mb4

[discomrade]

#193 Related?

Vichan has legacy code to manually extract the version number of MySQL to check if it's greater than or less than 5.5.3 (from 2010) and if it is less, make changes to the database on installation and to character encodings when inserting into the MySQL.
MariaDB versions can have version numbers that cannot be interpreted by this function, resulting in strange behavior.

My recommendation is:

• transforming our database to use utf8mb4 instead of utf8
• officially stop supporting MySQL < 5.5.3
• remove all references to mysql_version()

One manifestation of this bug is failing to edit posts with emoji in them.