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 local disk storage engine.
• Add a "storage engine selector" class which manages choosing which storage

engines to put files in.

• Document storage engines.
• Document file storage classes.

Test Plan:
Setup mode:

• Disabled MySQL storage engine, misconfigured it, configured it correctly.
• Disabled file storage engine, set it to something invalid, set it to

something valid.

• Verified max_allowed_packet is read correctly.

Application mode:

• Configured local file storage.
• Uploaded large and small files.
• Verified larger files were written to local storage.
• Verified smaller files were written to MySQL blob storage.

Documentation:

• Read documentation.

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, epriestley, jungejason
Differential Revision: 695