Add a way for users to contribute token overlays

[Azhrei]

If users want to contribute them, they would currently need to fork the repo and submit a PR. That's a lot of effort for someone who just wants to contribute some graphics assets back to the community.

Ideally, these assets would be stored on RPTools.net and TokenTool could check at launch for new assets and offer to download them (caching anything previously downloaded). Then contributing new ones amounts to adding an upload facility for RPTools.net, perhaps by a WP page that allows users to contribute and our Bard (RPTroll) to "accept" them for "publication" (i.e. approve them to be copied into the distribution directory where users can get them).

We'll need a policy that says we can reject images we don't consider in good taste, and we'll need support both on RPTools.net and in TokenTool...