Database Manager: create manual migration

Sometimes you need to create very specific migrations that are not covered in the UI.

This FR is to add a Create manual migration in the context menu of Changes.

It should open a template with the following contents or similar.

// For documentation on how to create migrations go to http://knexjs.org

exports.up = function (knex) {
  return knex...
};

exports.down = function (knex) {
  return knex...
};

On save it should add the appropiate naming convention and add the database info when processed.