[feature] Add optional clean insights data collection

[nemesifier]

Depends on #273

• schema:https://gitlab.com/cleaninsights/clean-insights-design/-/blob/master/schema-docs/README.md - we should focus on events
• info to collect:
  • openwisp modules enabled
  • version of the modules
  • OS version
  • available number of CPUs, total RAM
• in order to know if the data changes over time (needed to track upgrades), we need to store some data in the DB

We can collect this info when the application starts, maybe we should restrict this to the application server process only (no celery) and ensure it's performed only once, we should use a thread to avoid slowing down startup.

We need a setting to control whether this is enabled or not, it shall be disabled by default.

Let's add a section in the README which explains what data is collected and why.

We can test this in staging with the clean insights staging/prod system.