R1:97ff7fe259d4

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()`…