Add spell list downloading

[Dewyer]

Add a new feature that lets users download spells instead of choosing a local file to import.

• We should have a static gist that contains all the downloadable lists (links and names for them, this is what the app would first query to show what the user can download)
• Add a spell list thats useable for testing both this feature and the app in general (Would make development much easier)

In the future we could also add all the freely available spells from DndBeyond to a downloadable spell list.

Also by hosting xml files on github we could refrain from having to run our own servers for a while longer.