Show related contacts in the POI form

[MizukiTemma]

Short description

This PR implements a box into the POI form which shows contacts referring the POI.

Proposed changes

• Add a new collapsible side box like "Organization", "Barrier free" or "Actions"
• List up related contacts with a link to contact form respectively
• Add tests for this box

Side effects

• This brings more duplicate than mentioned in #3147 but it is consistent with other models (Organization, for example) to show the list of related objects. I'll leave the duplicate problem to #3147.

Resolved issues

Fixes: #3087

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