Treat messages as text

[mattfbacon]

SMTP messages, which sometimes appear as attachments, are currently treated as "possibly binary" by mox webmail. Since SMTP messages are ASCII, we should be able to treat them as text.

[mjl-]

Thanks, makes sense to show content immediately instead of requiring a click.

There is also still a todo in the list at the top of webmail/webmail.ts about just rendering a message inside webmail. In a popup, or simply below the regular email (not sure how we would prevent confusion about what the original message vs the forwarded message is).