`app.update.enabled` should remove updater UI elements when set to false.

I have a use case that requires setting app.update.enabled to false.

This mostly works as I expect, however the UI element for forcing a manual update check in the Help->About Tor Browser dialog still remains visible and functional. If the updater is disabled, the UI elements associated with it should be hidden.

On a side note, it would be nice if the built in updater could be force disabled at launch time via an env var as well, for those most special of snowflakes.