This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
* emails weren't sorted in correct order
* better default regex for splitting mbox files
* output Message-ID if email is skipped because it doesn't have a Date
* 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
* Disable journaling to improve performance in Docker
* Use the email cooking method
* Store IncomingEmail in order find related posts by Message-ID
* Escape HTML in imported messages
