fix: catch failed SENT copy operation

[miaulalala]

Sometimes, IMAP fails copying an already sent mail to the SENT mailbox.

In case this is not down to no SENT mailbox, we mark the local message and try again later, as this is often down to too many authentication attempts on IMAP.

To Do:

• [ ] BG job that tries copying the failed messages to the sent mailbox
• [ ] Possible frontend improvement that shows a message has been sent but couldn't be copied to the SENT mailbox. Thunderbird does this and lets the user decide if they want to try again - we could do that for Outbox mails as well.
• [ ] Tests