Make most file reads policy-aware

Summary: Ref T603. Swaps out most PhabricatorFile loads for PhabricatorFileQuery.

Test Plan:

• Viewed Differential changesets.
• Used file.info.
• Used file.download.
• Viewed a file.
• Deleted a file.
• Used /Fnnnn to access a file.
• Uploaded an image, verified a thumbnail generated.
• Created and edited a macro.
• Added a meme.
• Did old-school attach-a-file-to-a-task.
• Viewed a paste.
• Viewed a mock.
• Embedded a mock.
• Profiled a page.
• Parsed a commit with image files linked to a revision with image files.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7178