Feature: append code to settings.py

[mwallschlaeger]

Feature Description

Sometimes its required to manipulate or append configuration to the settings py. Most settings inside settings.py can be set via envvars and therefore using the values.yaml of geonode-k8s. But some changes are that specific that this helm chart cannot foresee them. Therefore adding something to the end of the settings.py can be very useful. E.g. to set social providers, install additional apps etc.