MemGen moved to a new domain

MemGen online-service have been moved to a new server and got a new domain.

Domain have been changed from http://memgen.uni-goettingen.de (inactive) to https://memgen.uni-saarland.de.

Software have been extracted from the previous server and placed in internal Git repository. New deployment is using Docker and GitLab CI.

Some cleaning, refactoring and update of dependencies have been performed. Operating system have been updated from native Ubuntu 14.04 to 20.04 LTS in a container.