Load dashboard on environment setup

[aspedrosa]

We already have some dashboards setup on our deployment.
This PR creates a script to upload, these already setup dashboards, to a superset installation. This script uses commands provided by the superset python package to execute these uploads.

Changelog:

1. Move materialized_queries_manager models to the achilles database.
2. Use only one migrate command for the achillesdatabase.
3. Create fixtures to insert initial data. The insertion of countries' initial data is made now through it, previously was made using a django-admin command.
4. Create a command to generate materialized queries records as materialized views on Postgres, to be executed after loading the initial data.
5. Static files of the dashboard app are now accessed by Nginx through a named volume instead of a bind volume.
6. Superset docker-init.sh script now only applies the migrations and creates roles and permissions. The creation of the admin user was moved to another script which will run only one time.
7. Created a separate script to load the already built dashboards (load-dashboards.sh).
8. Added a SUPERSET_URL variable to .env-example so that before tabs initial_data fixture is loaded, the value of this variable is used to complete the URLs present there.

[aspedrosa]

• flask shell run a python shell with the running app's context

• To insert new records:

from superset import db
db.session.add(model)