Refactor Task Recs experiment bucketing logic

* Add the TaskRecommendationsExperimentV1 class, which models a user
  being entered into the Task Recommendations experiment
* Use the class in Hooks::onBeforePageDisplay and Hooks::onPersonalUrls

Change-Id: Ia30ef2481147b78ef2126c328774145495051afc
Merge "Refactor Task Recs experiment bucketing logic"