For messages with a base MIME type of multipart/alternative, we were
hiding text/plain subparts based on the number of alternate subparts.
This assumption of course broke on a message with the following body
structure and a Plain Text Mode preference of "Show HTML if present":
multipart/alternative
text/plain
text/plain
Instead, note when we've actually seen a text/html subpart and use that
to decide whether to hide the text/plain parts.
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