Create separate git repository for MediaGoblin themes

[Jim Campbell]

MediaGoblin should provide a separate mediagoblin-themes repository similar to that which the Pelican project maintains [0].

It would be simple to set up, and would provide a common platform for theme hosting and user theme discovery. The core themes could be
maintained directly in the repository, and any user-contributed themes could be listed / provided as git submodules.

To be included, each theme should ideally include at least one screenshot of the theme as deployed and state which MG versions it is valid against.

[0] ​https://github.com/getpelican/pelican-themes

[Christopher Allan Webber]

Good idea, but I think this doesn't need to be tied to the 0.8.0 release especially since we're coming up close.