Migrate database over to MongoDB from MySQL

[pedropapa]

MongoDB should be much faster and more suitable for a gaming server than MySQL.

Sequelize should take care of Database differences and this database change should be smooth. I hope so.

Tasks

• Remove MySQL references on both code and docs.
• Make necessary tweaks to make MongoDB work with the current server code.

Notes: I don't think it's necessary to make database data migration since the server never went live.