Use viwer-based checks for application visibility when rendering home elements

Summary:
Fixes T4619. Currently, even if a viewer can't see Maniphest, they'll still see empty panels on the home page. These panels will always be empty so there's no real policy violation, but it's confusing.

Longer term, dashboards should fix this.

Test Plan: Viewed home page with a user with and without permissions on the apps.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4619

Differential Revision: https://secure.phabricator.com/D8545