Fix/ Venue: Custom Stage signatures and invitation_edit_process paper number

[celestemartinez]

Fixes #2083

This PR removes PCs from signatures of the CustomStage and edits how we get the paper number in invitation_edit_process.py
I am not sure what the best way to get the paper number is when we are deadling with reviews and metareviews, but this is a possible solution.

[melisabok]

I'm not fully convinced about checking if the note is a tuple or not.

[melisabok]

We could always return a tuple as [note, forumNote] where in the case of submissions they are the same note:

source_submissions = [(n, n) for s in client.get_all_notes(content={ 'venueid': venue_id }, sort='number:asc')]

and when we build the invitation edit content:

{
  noteId: forumNote.id,
  noteNumber: forumNote.number
}
 if 'replyto' in invitation.edit['content']:
        content['replyto'] = { 'value': note.id }

....

I think noteId and noteNumber should have been called forumId and forumNumber but I think it is too late.