R1:2b7210260f10

Allow Phabricator storage engines to be extended and configured

Summary:
See T344. Currently, there's a hard-coded 12MB filesize limit and some awkward
interactions with MySQL's max_allowed_packet. Make this system generally more
robust:

- Move the upload limit to configuration.
- Add setup steps which reconcile max_allowed_packet vs MySQL file storage
limits.
- Add a layer of indirection between uploading files and storage engines.
- Allow the definition of new storage engines.
- Define a…