Create temporary preference to disable VisualEditor during beta

This preference will allow users to opt-out of VisualEditor during the beta
if they so choose. We do not re-use the alpha enablement flag because (a) this
would lead to a confusing preference description, and (b) because opting in
and then out of the alpha is not the same user choice as opting out of the
beta period.

Change-Id: I0f0a1b5eb21703ad422d007cab65c75ac1aa6fd8