Feature: basic permissions system

This pull request adds basic permissions system. If you don't need this, you can safely ignore this. But for those of you who need, you can now define who can view only the data and who can create & execute queries.

For in-company re:dash installs, we highly recommend leaving this open to everyone. This is the spirit and vision behind re:dash -- making data accessible and open to everyone in your organization. Yet, this feature  enables you to use re:dash in scenarios where you might want to share parts of your data with external entities.