Icon Option for non swc/dc/lc workshops

Generally: Is it possible for the `_config.yaml` option for `carpentry:` to have an option for lessons which are not-dc/lc/swc. This could be useful for incubator lessons or other lessons folks use the template with.

Specifically: How can I change the upper icon to not be swc/dc/lc for a lesson that is using this template?

cc: @cmilica

Start with _config.yaml and trace how the carpentry option controls the upper icon in this lesson template. Done means a lesson using a non-dc/lc/swc value can select an appropriate upper icon, with the supported option documented for incubator and other lessons.

Written by the indexing model from the issue text.