Django 3.1 supports relative paths for static_url.
Use it to make it more flexible.
Minor fixes for upgrade:
* ignore-fail-on-non-empty for rmdir /usr/share/etesync-server/etesync_server
* do not stop service (it is stopped already and init file is removed)
Signed-off-by: Peter Stadler <peter.stadler@student.uibk.ac.at>