When sending an email via the mail.compose wizard, the selected partners are
stored in the context (`active_ids`).
If the composed message is saved (button "save template"), the context is lost
in the _reopen action.
The active_ids content of the new context is the id of the newly created mail
template and is used as the id of a res.partner (sending the email to a
different contact).
Keep the context during the reopen to avoid losing active_ids.
Closes#11947