Deprecated discourse-alt-logo is still needed for custom themes

Recently, the decision was made to deprecate discourse-alt-logo because it’s possible to set a dark logo:

This change has removed valuable functionality that supports the implementation of a per-theme logo. I use this functionality on a number of sites, e.g. https://forum.garudalinux.org.

Therefore, please consider “un-deprecating” this plugin. Thank you!

Just in case anyone else is using this plugin, I’ve reverted the final commits so Discourse doesn’t complain about the deprecation.