- refactor Network class

- catch IntegrityError on patch
- disable autoflush (IntegrityError was raised before commit due to
autoflush)

- use same postgres container for dev and test
  (create 2 different databases)
- use docker-compose.override.yml to avoid duplication
- ignore more files not needed in the docker image

By default docker-compose reads both docker-compose.yml and
docker-compose.override.yml.
In docker-compose.override.yml we mount the current directory as
a volume for development.

To test the image we only use docker-compose.yml:
- don't force port 5432 to the outside world to avoid conflict
- do not mount the current directory

Put under the functional directory all tests that require
access to the database. There is a "session" fixture in that
directory that is automatically invoked for each tests. It rollbacks
any transaction after the test to leave the database clean.

Other tests should go under the unit directory.

Inspired from:
- https://stackoverflow.com/a/38626139
- http://alexmic.net/flask-sqlalchemy-pytest/