fix: Upgrade to Django v4, docker image to bullseye.

See merge request !38

# Conflicts:
#   fdroid_website_search/urls.py
#   requirements.txt

fix: Added missing call to super.__init__, added timezone to repoTimestamp,...

See merge request !36

fix: Minor corrections; missing call to super.__init__, repoTimestamp timezone,  added timezone to repoTimestamp, error in template tag if site does not exist.

Add link to source and update copyright

See merge request !35