History tags: simpler algorithm

For each tag, over 30 days:
* each day with a visit = 10 points
* each additional visit in a day = 1 point
* weight today's tally by 30, yesterday's by 29, the day before yesterday's by 28, etc.
* no longer computes a "habit factor"; the sample size is too small to make that stable.
* omit anything already in the top nav
* omit profile/.. tags.

@crifmulholland