R1:c705c8011e96

Use PHP implementation of Cowsay for cowsay rule

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…