Introduce google spreadsheet crawler

[fcamblor]

Benefits :

• Better UX compared to JSON file (😍Data Validation constraints)
• Lower the barrier of entry (no need to be a dev)
• Inline hints
• GSheet can be easily shared

I created a Voxxrin template spreadsheet for this : https://docs.google.com/spreadsheets/d/1ulA9VuwWbSZucE8TQToyawC1GL-6JqQnpWCe1XeqP_M/edit?usp=sharing

JSON from this spreadsheet can be gathered through :
GET https://docs.google.com/spreadsheets/d/1ulA9VuwWbSZucE8TQToyawC1GL-6JqQnpWCe1XeqP_M/gviz/tq?tqx=out:json&tq&gid=1914487150
(this is JSONP-like)

If I can implement a data from gspreadsheet reader, this would allow to avoid implementing it right into Voxxrin and would make configuration far more flexible/agile.

[fcamblor]

It would also be a nice thing to allow to import a conference-hall event config into this spreadsheet, so that every conference-hall information is automatically gathered (only the timeslot definition remains)

This process would have to be idempotent (we should re-import conference-hall data multiple times without erasing provided timeslots)