Why this change?
This change allows plugins or themes to replace the tag icon in the
sidebar. The color of the icon can be customised as well.
However, do note that this change is marked experimental as we intend to
support custom icons for tags in the near term as part of Discourse core.
Therefore, the plugin API will become obsolete once that happens and we
are marking it experimental to avoid having to deprecate it.
e880ede3d8