Switching format keeps the original quote, not a flattened copy (#409) (#409)

Switching a reply between plain text and rich text converted whatever
body the draft was showing. Going from plain text to rich, that meant
the quoted message came back as the "> " text quote run through a
converter -- the sender's formatting, images and links gone, even though
the original markup was sitting on the draft untouched.

Both forms of the quote are prepared when the reply opens, so keep them
on the draft and re-attach the right one when the format changes. Only
what the author typed above the quote is converted. Where the quote
can't be found any more -- edited by hand, or a draft that quotes
nothing -- the whole body is converted as before, which is what every
non-reply draft does.

No new strings.

web/src/store/__tests__/compose-email.test.ts

@@ -18,7 +18,7 @@ function draft(over: Partial<Draft> = {}): Draft {
     requestReceipt: false, priority: "normal",
     showCc: false, showBcc: false, showReplyTo: false,
     minimized: false, maximized: false, dirty: false, savedAt: null,
-    saving: false, sending: false, error: null, signatureHtml: "", replyMode: null, formatOffer: null, sendAt: null,
+    saving: false, sending: false, error: null, signatureHtml: "", replyMode: null, quoteHtml: "", quoteText: "", formatOffer: null, sendAt: null,
     ...over,
   };
 }