Support deleting individual changeset

Summary:
add support in purge_cache.php. We might want to add support
for maniphest comments and differential comments later.

Test Plan:

• purging all worked
• purging individual changeset worked
• when input is not integer, correct error message reported

Reviewers: epriestley, btrahan, nh

Reviewed By: nh

CC: nh, aran, jungejason, epriestley

Maniphest Tasks: T683

Differential Revision: https://secure.phabricator.com/D1372