RMST-388: Weekly cronjob to report consumption movers on Slack

[leplatrem]

The idea is to send a weekly post on Slack that will give a summary of top cost increase and decrease

*Bandwidth Consumption*
Period: Apr 26 → May 3

*Top increases* (Compared against previous 90-day daily average.)
- `crash-reports-ondemand`: +629.1%
- `startup`: +12.1%
- `nimbus-desktop-experiments`: +8.6%

*Top decreases*
- `quicksuggest-amp`: -91.2%
- `cert-revocations`: -53.6%
- `intermediates`: -36.6%
- `newtab-wallpapers-v2`: -63.2%
- `translations-models-v2`: -25.7%