Bug: import/archive don't unfold headers before storage in ES

The mail parser returns the headers as-is, including line-wraps.

This is what is wanted for the raw email source, but is not really suitable for fields such as In-Reply-To, References etc.

It seems there is no unfold method in the Python email or mailbox modules, so it looks like it is necessary to write one.

Header values that have not been folded cannot contain CRLF, so unfolding should just be a matter of stripping these out. There should be no need to check if the CRLF is followed by whitespace (assuming the folding has been done correctly).

It might make sense to compress runs of WS to a single space in case there is some variation.
This should make it easier to match things like In-Reply-To.

Trace the import/archive path that writes headers to ES, then inspect the Python email and mailbox modules' handling of folded headers. Done means fields such as In-Reply-To and References are unfolded before storage while the raw email source remains unchanged; verify that matching works with folded input.

Written by the indexing model from the issue text.