split from #8516

Some options here:

• Allow the addon.icons property to be writeable and for an upload blob to be specified (null for delete, I guess) <- going with this option
• Have a specified sub-endpoint under addons to upload an icon and/or delete it
• Support updating the addon icon by uploading a new version of the xpi - i.e. extract the icon from the xpi for each version. (if we're going to do this automatically, we probably don't want to allow other ways for the icon to be updated)