Consider making richer HTML emails

[spbnick]

We could make HTML emails much more useful than plain-text emails, while keeping the plain-text look by adding links from some items, like builds and tests. However, it would be good to avoid requiring subscriptions to format an additional version of their message. Perhaps we could format Markdown or a similar language instead, and then render it automatically into the two versions? One fixed-width HTML with links, another plain-text without links (apart from those directly in the text).