[Watcher] Support icon_emoji

[elasticmachine]

Original comment by @sherry-ger:

The slack API supports icon_emoji. Please support icon_emoji from watcher actions not just icon as it is available now.

[dakrone]

This has been open for quite a while, and hasn't had a lot of interest. For now I'm going to close this as something we aren't planning on implementing. We can re-open it later if needed.