Skip rare stacks for duration-based sorts.

Outliers were crowding up the place. Skipping stacks we've seen
less than N times shows more interesting stacks.
Split "combine week's data" into a separate script
Remove "combine weekly files" code from run_public
Merge branch 'chromehangs_skip_duration_outliers'
Merge branch 'master' of github.com:mozilla/telemetry-server