Bug: drop 'archived-at' header generation

Currently the archiver adds an 'archived-at' header to all messages.
This changes the source, and can cause the message to get a different id when reloaded.

The arrival time is still useful, but it should be stored as meta-data in the ES database rather than by changing the source.

Note that the header does not appears in mails stored by lists.apache.org since early Jan 2018, judging by the sources of dev@ponymail

Start by locating the archiver entry point that adds the 'archived-at' header and trace how arrival time is persisted in the ES database. Done means archived message sources no longer gain that header, message IDs remain stable when reloaded, and arrival time is still available as metadata.

Written by the indexing model from the issue text.