Create dependencies Docker image

Ghost User
2020-02-08T22:06:54+01:00

### Description The goal of this MR is to create a new Docker image that contains all the dependencies of Concrexit. This image would be built using a Gitlab CI Pipeline schedule. With this change the following would happen: - Every day (or on some other schedule) a new Docker image containing only the dependencies (including the development dependencies) is being build. - The CI jobs run using the dependencies image. This removes the dependency install time from every job. - `poetry install` is still being run on every job, to make sure dependencies specific to that branch are being installed. - A Docker image (`concrexit:`) is being build on every branch, but significantly faster because the dependencies image is used as the base image. - When a production Docker is being built, a new dependencies image without the development dependencies is build first. The advantages: - Faster builds. - Always build using up to date dependencies ([thaliawww/python-thalia](https://github.com/thaliawww/python-thalia) is not automatically build and updated). - Better maintainable configuration, because everything is in this repo instead of two repos on two different platforms. Because a lot of refactoring is being done, this MR also refactors the whole config to be consistent.