Summary: Ref T7785. Convert the Cowsay Remarkup rule to use a PHP implementation so we don't have to execute an external `cowsay` binary. I removed some of the default ".cow" files that come with Cowsay because they: - include Perl code which we can not interpret; or - are primarily in-jokes or standalone visual puns or artwork rather than usable actors on the grand stage of cowsay; or - offended my delicate sensibilities. Users can add new cows to `resources/cows/custom/` if they want to make new cows available. I have included a majestic original artwork depicting the "Companion Cube" character from //Portal//. Test Plan: {F802535} Reviewers: chad Reviewed By: chad Maniphest Tasks: T9408, T7785 Differential Revision: https://secure.phabricator.com/D14100