Feature/storage/monitoring tool

[OhMaley]

Description

This PR aims to offer a monitoring tool similar to the one available in Codalab.
The following features have been developed:

• The monitoring tool is only accessible by admins
• It is accessible under the user menu -> analytics -> storage tab
• 3 metrics are available
  • Usage history
  • Competitions usage history and details
  • Users usage history and details
• Metrics can be download using the download buttons
• The metrics only displays what has been computed by the server beforehand
• 2 background tasks has been coded:
  • a reset task that get executed every 3 month (based on the value that is set in the settings/base.py). It reset the value of all computed file sizes so they will be re-computed again without any shifting on the next run of the storage analytics task
  • a storage analytic snapshot task that get executed every week (based on the value that is set in the settings/base.py). It measures all files with unset size. Then it measures the storage usage per category (competition, user or admin) and per day. It then check for database <-> storage inconsistency (and created a log file in the logs/ folder). And save everything into the database so it becomes query-able by the front-end.

Issues this PR resolves

#713

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge

[Didayolo]

Should we merge this to a new branch on codabench repository to trigger the automatic tests?