Make file upload policies more consistent

Summary:
Ref T7149. Currently, global drag and drop always uses the most open visibility policy on the install. This was appropriate before the application preference was introduced, but default to the application preference now.

In particular, this supports a default value of "Administrators" in the Phacility cluster.

Also simplify/clean up some code.

Test Plan:

• Set application default policy to "Adminstrators".
• Uploaded file via drag-and-drop, saw "administrators" policy.
• Uploaded file via arc upload, saw "administrators" policy.
  • Saw better URI for a text file upload after patch.
• Uploaded file via drag-and-drop-to-textarea, saw "only viewer" policy.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12093