Preserve the state of the per-transaction squelching checkboxes

Now unchecked recipients are preserved across attachment uploads,
article inclusions, and any other action which submits the form but
doesn't submit the update.

Refactors the code to process squelched recipients for use outside of
Ticket/Update.html.

Resolves #18761.
Merge branch '4.0/preserve-txn-squelching' into 4.0-trunk