[generator] Phase 2: Don't check in autogenerated content into Git

[luxas]

Continues on the effort of making the generator more developer-friendly :smile: Previous PR was https://github.com/cloud-native-nordics/meetups/pull/92

This PR:

• Simplifies the code a bit (many PRs needed for the end goal here sigh)
• Ditches to check in autogenerated content; it's fetched (in parallel) upon starting the program
• Unifies make and make stats so they don't become out of sync; now stats.json is always performed upon running make
• Adds the meetup group's photo and description in the types, something we can use later

cc @kaspernissen @netumoliver

[kaspernissen]

I will merge this now.