Add anonymized attendee stats

[Mariatta]

Now that registration already opens we can begin collecting data and displaying anonymized stats of attendee demographics.

• number of registrations
• grouping of attendees by role, job, location, experience level, language, etc

What needs to happen:

• Create models representing attendee data
• add webhook to receive notification that there is new registration on pretix
• call pretix api to collect up to date data
• perform calculations
• display the data in the stats page and in the json endpoint