Deleting an Initiative/user should also delete its associated social links

[dulldesk]

May need to come after #70 is implemented

[fluix-dev]

Shouldn't need to come after #70, that will touch the SocialLink, while this only affects the SocialLinkAttachement.

[dulldesk]

This is probably relevant

Unlike ForeignKey, GenericForeignKey does not accept an on_delete argument to customize this behavior; if desired, you can avoid the cascade-deletion by not using GenericRelation, and alternate behavior can be provided via the pre_delete signal.

source

[fluix-dev]

So the solution (it seems, though not explicitly stated in the docs) is either using GenericRelation or pre_delete. GenericRelation might actually be quite handy for simplifying our SocialLinkAttachment queries.