Change the encoding of the database from utf8 to utf8mb4

[cedricbonhomme]

• https://mathiasbynens.be/notes/mysql-utf8mb4
• https://medium.com/@adamhooper/in-mysql-never-use-utf8-use-utf8mb4-11761243e434
• https://dev.mysql.com/doc/refman/5.7/en/charset-unicode-utf8mb4.html

MySQL utf8 encoding is broken and dangerous for security reasons. The switch seems easy and could be done in the context of developments for the issue #7.