Improving Topics assignment for better utility

[lbdvt]

Topics are assigned to users based on the most accumulated ones during the considered epoch. This tends to favor the same generic and popular topics for most users (e.g. 'news').

To increase the utility of Topics, it may be helpful to consider first normalizing the accumulated list of topics using the relative frequency of all topics in the user's country or region, and then selecting the top 5 topics.

[jkarlin]

There is related discussion to this in #42, though that one doesn't mention region. Okay if we close this one in favor of consolidating discussion? If not, feel free to open back up.