Events table schema update

[yakkomajuri]

We need to update our events table to the new schema that has all the required persons/groups data.

We'll need the following columns:

• person_id
• person_properties
• group0_properties
• group1_properties
• group2_properties
• group3_properties
• group4_properties

This should be an async migration that will depend on the sharding migration.

We probably want to do this manually on Cloud at first before adding a migration for self-hosted users.

For self-hosted users we might also want to batch the table updates with the backfill.