Implement CSV export

• Adjust field types
• Rename app
• Correct column names
• What is with those specific process field types?
• Write some converters (converters with args? → date format)
• Photo files
• Simple CSV template

mentioned in merge request !115

changed the description

changed the description

assigned to @hansegucker

added part::backend type::feature labels

added part::frontend label

marked the checklist item Adjust field types as completed

marked the checklist item Correct column names as completed

marked the checklist item What is with those specific process field types? as completed

marked the checklist item Simple CSV template as completed

Seemingly, UUIDs are generated on import, which leads to an issue in the following case:

Person "Albus" already existing in AlekSIS -> Export (CSV contains "Albus" without UUID") -> Import -> Two Persons "Albus"