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
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.
Although 407a23663d will send rejection
messages for unrecognized errors, sometimes processing the email will
raise an error which has a blank message.
This commit:
1. Shows rejected emails which have already been processed and contain
a blank error in /admin/email/rejected
2. Replaces new blank error messages with the error type
