The `oninput` event is a more performant way to detect changes to a
textbox’s contents (compared to `onkeydown`, `onpaste`, etc).
It’s not supported in older browsers, but since this is a likely to be
used by platform admin users only that’s OK.
This commit improves the code that previews a hex colour when setting up
or changing an email branding.
Specifically it:
- refactors the Javascript to conform to our patterns (module pattern,
preprocessed with Gulp)
- makes the code work when there are multiple colour previews on one
page
It also does some visual prettifying, because I couldn’t help myself…
