Add a stats endpoint

[virgile-dev]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
Every week we have to ask a devops to give us the number of docs and users.

Describe the solution you'd like
I'd like to have a /stats that exposes in json a few stats.

• Number of users
• Number of docs
• Number of active users / per week
• Number of active docs / per week

This will be usefull for the overall dashboard of La Suite.

We can refine what stats we display, for example

• Number of shared Docs
• Pie chart of different sharing settings (public, authenticated, private)
• Medium number of users on a doc

[lindenb1]

@virgile-dev Feature PR has been created accordingly, if there are more Metrics you need, please let me know.

-> #449