When showing policy hints, if the Space policy is strictly stronger, show it instead

Summary:
Ref T8449. Before we show a policy hint in the header of an object, compare it to the space policy (if one exists).

If the space policy is strictly stronger (more restrictive -- for example, the Space policy is 'members of X', and the object policy is 'public'), show the space policy instead.

See discussion on T8376.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8449

Differential Revision: https://secure.phabricator.com/D13328