allow to delete contact details

[tttp]

It might be useful to empower the organisations to delete themselves contact details in the following contexts:

• specific signature/supporter (to comply with GDPR request to forget)
• specific campaigns (eg if the campaign is finished and the export is done, no need to keep the data anymore and no need to wait for the normal deletion after x months)

these both should be - obviously - authenticated and only available to the org owners

the second one might me safer for now to do keep as a cli only call