Load custom CSS for KB

[MrGeneration]

Infos:

• Used Zammad version: 3.0
• Installation method (source, package, ..): any
• Operating system: any
• Database + version: any
• Elasticsearch version: any
• Browser + version: any

Desired behavior:

It should be possible to load custom CSS to the knowledgebase.
This should be done similar to how we do it on our main application via app/assets/stylesheets/custom/ .

Currently you don't have any chance to manipulate e.g. header links in any way. (see: #2619 )

[pgassmann]

We also want to change the HTML of the KB to have the same design as the Website.
With zammad in docker-compose we can do this now by mounting the app/views/layouts/knowledge_base.html.erb on the zammad-railsserver container from outside.

  zammad-railsserver:
    command: ["zammad-railsserver"]
    depends_on:
      - zammad-memcached
      - zammad-postgresql
    image: zammad/zammad-docker-compose:zammad-{{ zammad_version }}
    restart: always
    volumes:
      - ./data/zammad-data:/opt/zammad
      - type: bind
        source: ./knowledge_base.html.erb
        target: /opt/zammad/app/views/layouts/knowledge_base.html.erb
        read_only: true