Adding build tag on jobs

See merge request !3

Update the license

See merge request !2

Development

See merge request !1

# Conflicts:
#   .gitlab-ci.yml

Attempting to get the User-Agent within a try-except block so as to make the service usable even without the User-Agent header

- Loading the BackgroundScheduler independently from Flask.
- Using the --preload parameter in Gunicorn, which makes sure that the BackgroundScheduler is launched only once (instead of once per worker).