Add 'HHVM' beta feature

Allow users to opt-in to HHVM beta testing via the preference interface
provided by the BetaFeatures extension. The "HHVM" cookie is set
upon login and after saving preferences. Additionally, the page
load time and the request backend (HHVM or PHP5) will be shown
in the personal tools menu.

Change-Id: Id9e7dbbac5bd142bbe187c4d73392ecc79643325
(cherry picked from commit 08816f2108530a4c1c1c83b4a9958c8e78cb46cf)