Show CalDAV/CardDAV tab when the respective service is present

[rfc2822]

Purpose

Previously, we showed the

• CardDAV tab when there's at least one address book (or if there are null address books, which happens when there is no CardDAV service, which caused CardDAV tab shows without detected resources  #864).
• Same for CalDAV.

Short description

This PR uses the existence of a CalDAV/CardDAV service as a condition for whether to show the respective tab.

This may cause confusion for users who use servers that report a CalDAV/CardDAV service without providing a Web interface for it. However, they can sometimes still create a collection, which may be want they want.

An alternative would be to just check for null and then hide the tab, too. However as said above, they may then never be able to create the first collection on an empty server.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[sunkup]

Works 👍