Use sequelize migrations

[eliihen]

Instead of automatically creating tables when the application starts, we should use the sequelize migrations functionality. If we do this it will be much easier for users to migrate from one version to another while retaining their data, and much easier for us to add new fields and tables.

Consider throwing a descriptive error on application start with details on how to run the migrations if the administrator has not yet done so.

http://docs.sequelizejs.com/manual/tutorial/migrations.html