Add gemini:// support

Since our hackerspace is now proudly part of the Geminispace over at gemini://coredump.ch/ I'd like to expose that fact over the SpaceAPI. I saw that we have a `gopher` key in the `contact` object so we could probably add it there.

But this is kind of hidden and we have a `url` top-level key for http URLs, which seems an unfair favoritism towards the web vs gopher and gemini.