Problem: It's not very easy to change the logo in the web UI

[weex]

As an instance admin, I might want to upload a new logo that would help my community better identify and enjoy using the instance, but that is currently quite hard to do.

Note: We've partially changed the default logo which requires specifically-formatted SVGs that must be placed in various locations. Logos take multiple forms as an icon, a light background, dark background, icon with an overlay of a logo that says "mastodon" so this is non-trivial.

Potential small step forward : Make the process that we use for the favicon work for just one other logo position.