Rename `bin/files metadata` to `bin/files rebuild` and let it rebuild MIME information

Summary:
See <https://github.com/facebook/phabricator/issues/320>. Files can end up with a bad MIME type, and we don't update it when uploading another copy of the file since obviously the new copy has the same data and thus the same MIME type.

  - Rename `bin/files metadata` to `bin/files rebuild` to make it a more consistent verb.
  - Let it rebuild MIME types so users who hit issues like this can run `bin/files rebuild --all --rebuild-mime` to straighten things out.

Test Plan: Ran `bin/files` in various modes, examined output.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5951