Allow simple template-based skin definitions

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