Overhaul & rewrite serialization of Recent Activity

[jvyden]

This splits things up to work like the rest of the API.

Key points:

• Proper separation between API and Game data types
• Most logic is no longer tangled in ActivityPage
• APIv3 now has access to groups (we should remove the events array in APIv4)
• Grouping is now done as properly intended
• Pagination in-game should be a bit better
• This is a bit more performant since we truncate to the last 6 months by default, but work still needs to be done after Postgres.
• Activity generation is now handled entirely in Refresh.Database