MySQL-compatible migrations

Make migrations run on MySQL, let Rails handle cross-DB stuff

The `limit: 191` on indexed string fields is due to [a mysql
limitation](http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-upgrading.html):

> InnoDB has a maximum index length of 767 bytes, so for utf8 or utf8mb4
> columns, you can index a maximum of 255 or 191 characters,
> respectively.

`utf8mb4` is required for emoji support, thus 191.