Message display corruption

[JayDaley]

Scrolling down though the messages to the dnsop mailing list using the down key I got to this one and then after that, each time I pressed down key the bottom pane showing the message did not change but linked text "Show header" was added, with the link to https://mailarchive.ietf.org/arch/browse/dnsop/#. This is reproducible.

See screenshot:

[rpcross]

This is caused by recent rate limit rules applied to mailarchive.ietf.org to combat abuse, 40 requests per minute per IP. The down arrow loads content for every message viewed in the preview pane. The UI needs to be refactored to load content more efficiently.

[rpcross]

Related to issue #3859