Summary:
Lower the barrier to entry for installing and creating skins, so we can kill Wordpress. You can now install skins by dropping them into a directory, and build either "advanced" (full phutil library) skins or "basic" (simple PHP templates) skins.
Next up is getting static resources working in an easy way for skins.
I put these in `externals/` for now so they don't get hit by lint.
Test Plan: Viewed the Pokeblog with the Oblivious skin.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T1373
Differential Revision: https://secure.phabricator.com/D3717
