Fix SchleuderResend script after conversion to WebExtension API

[doobry-systemli]

• fix(SchleuderResend): Fix parsing body of compose message
• fix(SchleuderResend): Always return a 'X-RESEND' pseudo header

[jobisoft]

I did not see this PR earlier, sorry. Can you explain in a few words, what this PR does and why it is needed?

[doobry-systemli]

It fixes the SchleuderResend script. Schleuder is an encrypting mailing list manager. Using pseudo-headers in mail body (e.g. X-RESEND: account@example.org), one can define custom schleuder actions. Is this enough context, or do you have further questions? ☺️

[jobisoft]

There is a conceptional change in the patch. The original script searched the message we reply to. The new script searches the message currently being edited. I simply do not understand why this is done. But I also have absolutely no idea how Schleuder works, so ...