Add product analytics

[chrean]

Currently, figuring out what Solidus needs to evolve is made harder because there are not enough data.
In addition to recurring user interviews, we plan to add telemetry (product analytics) to Solidus, to better understand how users engage with the software.

Product Analytics must be:

• Anonymous. We don't care about the who, only the what.
• Optional. Users need to be informed and can opt out if they don't wish to share any information to improve the product.

There are some challenges to face and choices to be made, e.g., where are data stored, who can access them, etc.

This is just a proposal, for now.