Links with icons to toggle between two states and show a spinner in between.
The link send POST and DELETE requests depending on the state.

Usage:

set data-toggle on a link like this

link_to label, url,
  remote: true,
  icon: :current
  data: { toggle: { current_icon_class: other_icon_class } },
  method: :post

When the link is clicked the current_icon_class will change to spinner_icon.
Once the request returns it will...
 * switch to other_icon_class if the request was successful
 * return to current_icon_class if the request failed.

It will also switch the method on successful clicks so it alternates between
create and destroy actions

This only uses browser js - it does not rely on prototype to ease the
migration to jquery (which it also does not rely on).
In order to enable this without altering class strings in plain js we
use the classList property of HTML5. This is present in all recent
browsers but IE. So for IE we have a shim.