Missing icon for WebDav

[danirabbit]

What Happened

When adding a CalDav server, the new account is created as a collection with the icon name onlineaccounts-webdav. This icon name is missing

Expected Behavior

We should probably remove the check for E.SOURCE_EXTENSION_CALENDAR and add an icon for onlineaccounts-webdav

Steps to Reproduce

1. Add a CalDav server
2. See that the added account has the default/fallback icon

[danirabbit]

Alternatively, maybe this icon is fine for webdav? Maybe it's the popover menuitem that should be updated? Can we add a contacts server here?