Make "isInstalled()" respect beta apps

Summary:
Currently, isInstalled() and getAllInstalledApplications() are inconsistent:

• isInstalled() returns true for beta apps, even if phabricator.show-beta-applications is false.
• getAllInstalledApplications() omits beta apps if phabricator.show-beta-applications is false.

Making the beta config control installs (not just homepage visibility) makes far more sense as we roll out more thorough application integrations.

Make isInstalled() respect beta, and clean up some callsites.

D5602 builds on this.

Test Plan: Installed/uninstalled beta apps, verified Conpherence menu/panel and other application integrations dropped out of the UI.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D5603