Move sprite sources from webroot/ to resources/

Summary:
  - Since we'll never serve these directly, move them to resources/. This makes generating the Celerity map faster and reduces the size of the result map, since we don't need to analyze resources we'll never serve.
  - Also Rename the 2x `subscribe-remove` to `subscribe-delete` since they were named inconsistently. Everything else is in good shape.

Test Plan: Generated sprites as per D4025

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2013

Differential Revision: https://secure.phabricator.com/D4026

Author: epriestley