Venue: let users delete invite assignments edges

[melisabok]

This way they can cancel an invitation to an emergency reviewer before accepting it.

[celestemartinez]

We should:

• add ACs (and SACs?) as writers of Invite_Assignment invitations
• check in the preprocess function if the reviewer has accepted the invitation. If they have, throw an error saying the edge can't be deleted
• if invite assignment is still pending, the AC can delete the edge and cancel the invitation

[celestemartinez]

Done here: #1999