mysql changes the type of TEXT columns to MEDIUMTEXT with "CONVERT TO",
but we don't want this, so change this columns back to TEXT.
see: https://bugs.mysql.com/bug.php?id=31291
Add warning about MySQL collation
Fix database index length
This allows new databases to be created with utf8mb4, on MySQL. The maximum
column size is 767 bytes. Each character is 4 bytes long -> 767 / 4 = 191
characters for the column.