Ignores the site setting "find_related_post_with_key" and always tries to honor the `In-Reply-To` and `References` header for emails sent to a group.
The senders email address must be included in the `To` or `CC` header of a previous email sent to the group and the `Message-ID` of that email must be included in the current email's `In-Reply-To` or `References` header.
* store time it took to index message in DB (to find performance issues)
* ignore listserv specific files
* better examples for split_regex
* first email in mbox shouldn't contain the split string
* always lock the DB in exclusive mode
* save email within transaction
* messages can be grouped by subject and use original order (for Listserv)
* adds option to index emails without running the import
* ignores dot-files and empty emails
* new setting to prefer HTML over plaintext emails during import
* restore original site settings at the end of import
* elided content of HTML mails was not put inside details block
