allow configuring the landing page

[EmTeedee]

Not sure this was wontfix for a reason other than no interest. Renamed the branch in line with contributing guidelines.

Resolves #766

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[EmTeedee]

CLA signed

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[nilmerg]

If we're going to merge this, then only by making it configurable by envvar. This is only an edge-case configuration which most of our users don't need. And if it is relevant to someone, it doesn't change often. (or not at all)

So please fetch the option from the envvar ICINGAWEB_LANDING_PAGE and drop the UI integration.

[EmTeedee]

@nilmerg I've made the requested changes and added documentation.