Identify what metrics we want to track for Early Bird

For the Earlybird launch, we'd like to have product dashboards using the events we send to posthog. Let's aim for a small starting point. Maybe:

For all services:

• DAU
• Unique visitors
• New users

For Thundermail, per day:

• Total emails sent

For Send:

• Total files sent
• Total bytes uploaded
• Total bytes downloaded

For Appointment:

• Total appointments requested
• Total hits to booking pages

@Sancus @radishmouse @kewisch please comment with your thoughts on these as the first metrics we focus on for product