backend: rework nginx config
backend: rework nginx config

Redesign entire nginx config to make it more readable. Also introduce
more security measures and DDoS prevention. The DDoS seems to be good
enough at this point and imho cannot be improved without introducing
complicated and new components.

closes #52

Signed-off-by: Marco Neumann <3829486b93ec44395f0b980424bae9b6fb07b7bc@crepererum.net>

spz-signup/spz-signup