WIP: Use a new table for user contribution stats

What does this MR do?

It defers the user contribution stats to a new table, instead of the events table.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This will allow down sizing the events table.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#22623 (moved)

app/models/user.rb

@@ -93,6 +93,7 @@ class User < ActiveRecord::Base
   has_many :todos,                    dependent: :destroy
   has_many :notification_settings,    dependent: :destroy
   has_many :award_emoji,              dependent: :destroy
+  has_many :user_contributions,       dependent: :destroy
 
   #
   # Validations