User invitations

Follow up from #331 (closed)

• Guarding registration with an invitation code and/or invitation link (preferably both) (!450 (merged))
• Linking invitation codes/links to an existing person to invite exactly this one person (!450 (merged))

added part::backend part::frontend source::customer type::feature workflow::confirmed labels

mentioned in issue #331 (closed)

mentioned in merge request !450 (merged)

marked the checklist item Linking invitation codes/links to an existing person to invite exactly this one person (!450 (merged)) as incomplete

marked the checklist item Linking invitation codes/links to an existing person to invite exactly this one person (!450 (merged)) as completed

closed with merge request !450 (merged)

mentioned in commit 2a7cc13a