Add a GUI for setting the value of dav.invitation_link_recipients, needed by non-public nextcloud servers

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Is your feature request related to a problem? Please describe.

The GUI doesn't provide any way to set/change the invitation_link_recipients dav apps setting.

Describe the solution you'd like

Idea: In the settings/admin/groupware endpoint, right below the setting send invitations to attendees toggle, when it is enabled, enable a toggle like " include web links (e.g. this server is accessible from the public internet)". By default it is on. When turned off, invitation_link_recipients could be set either to off or to system:mail_domain (that assumes a lot, but might be generally correct).

Describe alternatives you've considered
Document the occ command to be used to set the config variable when the nextcloud server instance is not accessible from the public internet.

Additional context

The dav app setting invitation_link_recipients (introduced in https://github.com/nextcloud/server/commit/31a25dc6b0545eb9bcb215990c8a71f1792f61f8) is used to suppress accept/decline web links in invitation emails, which is necessary for non-public-facing nextcloud instances where a user can send an IMIP invitation to a recipient outside their internal domain. invitation_link_recipients can be set to the name of the internal domain, or just on/off (to globally include/exclude accept/decline links

Of course, such web links in the invitation cause errors for such a recipient, when the sender's private nextcloud instance isn't accessible to external recipients. (Such recipients can of course still accept/decline if sender and recipient mail/calendar clients supports IMIP replies via email).

The (newer) /settings/admin/groupware allows the user to enable or disable "send invitations to attendees", but when enabled it should perhaps also have a toggle to suppress accept/decline web links (either asking for the internal domain, or even just on/off) for external invitation recipients from a sender using a non-public facing nextcloud instance.

Start at the newer /settings/admin/groupware endpoint and inspect how the existing “send invitations to attendees” toggle is wired to DAV settings. Trace the invitation_link_recipients setting and its occ configuration, then define the GUI behavior for non-public servers and verify the resulting invitation links for enabled and disabled cases.