In preparation for Jenkins PR 5778 I'm proposing a bunch of PRs for a variety of plugins that still look for paths rather than icon classes.

Skimming over email ext template, I noticed that there are are some GIF references in place that need to be replaced with icon references to prevent icons missing from the UI.
I don't know whether I have time to squash in a PR for this plugin as well but I'll drop the task here, in case basil or someone else wants to pick it up.