Popular tags - personal ranking

* A fancier - perhaps mildly baffling - algorithm for calculating a user's favourite tags. Takes into account "habit", a rough calculation of how consistent their pattern of visits is across 30 days.

* Separates logging of history from logging of tags; only logs history for video views (to support the requirement that autoplay only happens on a user's first visit to a video // @rich-nguyen )

@crifmulholland 

* Older days' weights fade linearly to zero, a few lines above:  `* (summaryPeriodDays - daysAgo)`
* `dayScore` says that, for a given day, the score is 1 + (number of visits that day)/5, with a cap of 20 visits. 
* all this is further weighted by habit factor - the consistency of intervals between days, where 0 means total consistency.