Enable linkchecker for POIs, events and contacts

[MizukiTemma]

Motivation

Contacts contain an E-mail address, telephone number and website, and POIs have an appointment URL. and Such elements are currently scanned by link checker and shown in the broken link list if they are referred in contents as link. We should probably scan contacts and events too.

Proposed Solution

Include E-mal address, telephone number and website of contact in the link checker. Include appointment URL of POI in the link checker.

Alternatives

Leave it unchecked

User Story

Additional Context

#2985 will help as example.

Design Requirements