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