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