Feature/allow custom stage rejected submissions

[melisabok]

Deprecate the source string: "all_submissions", "accepted_submissions", "public_submissions" and "flagged_for_ethics_review", "reply_to" and "source_submission_query". Instead use a source dictionary that allows several keys:

{
venueid: ['ICLR.cc/2025/Conference/Submission', 'ICLR.cc/2025/Conference/Submission/Desk_Rejected_Submission'],
readers: ['everyone'],
reply_to: 'Official_Review',
content: {
'flagged_for_ethics_review': True
},
with_decision_accept: True
}

each of these keywords are optionals, default value is { venueid: "venue_id/Submission"}

This is backward compatible but we should stop using source as a string and reply_to outside of source.

Tools module has a function to translate the source to expected format and two ways to create child invitations: for forums and replyto. This is reduce the duplicated code and have the source check in a single place.

CustomStage class can receive a source dictionary as parameter so we can specify more than one venue id.

Fixes: #2594 and #2599