Make "About" and "Help" sections configurable

[wetzel-desy]

Would it be possible to make the "About" and "Help" sections configurable with text in the frontendconfig.json? I would like to include a link to a data privacy policy and contact info for help with scicat in an instance of scicat.

[sbliven]

#1758 Starts on this by extracting the text content to config variables.

I wonder we could make this even more flexible with a template-based approach to configuration, where sites can supply and HTML template for the help pages. This would reduce the number of config variables needed and increase the flexibility, for instance allowing new sections to be added without rebuilding the image.