Send the read receipt the sender asked for

JMAP has an extension for this -- RFC 9007's MDN/send -- and Stalwart does
not implement it, so ihasmail assembles the RFC 8098 multipart/report itself
and sends it the long way round: raw MIME uploaded as a blob, imported,
submitted. That is also why the receipt lands in Sent, which is where it
honestly belongs.

The plumbing is the easy half. A receipt tells whoever asked that the address
is live and when the message was read, to an address the sender chose, so the
refusals are the feature: nothing marked Auto-Submitted (RFC 3834, or two
servers answer each other forever), nothing carrying Precedence bulk/list/junk
or a List-Id, nothing already acknowledged, nothing that never arrived. A
receipt aimed anywhere other than the sender is offered, but says so first.
There is no "always send" setting, only ask or never.

Sending is recorded with RFC 3503's $mdnsent keyword on the original rather
than remembered locally, so a second look -- or another client entirely --
knows not to ask again. Non-ASCII parts go base64 rather than 8bit, so
nothing rests on 8BITMIME surviving every hop.

Verified against the mock end to end: the blob uploads, the receipt imports
and submits, and the original reads back marked. Not yet exercised against
the live server.

web/src/jmap/types.ts

@@ -241,6 +241,8 @@ export interface Email {
   "header:X-Priority:asText"?: string | null;
   "header:Importance:asText"?: string | null;
   "header:Auto-Submitted:asText"?: string | null;
+  /** Bulk/list mail marks itself here; read receipts for it only confirm the address. */
+  "header:Precedence:asText"?: string | null;
   "header:Return-Path:asText"?: string | null;
   "header:Authentication-Results:asText"?: string | null;
   "header:Received:asText:all"?: string[] | null;