feat: Platform API improvements (#2900)

- Platform APIs to add and update custom attributes to users
- Platform APIs to delete accounts
- Platform APIs to delete users

db/migrate/20210827120929_add_custom_attributes_to_user.rb

@@ -0,0 +1,5 @@
+class AddCustomAttributesToUser < ActiveRecord::Migration[6.1]
+  def change
+    add_column :users, :custom_attributes, :jsonb, default: {}
+  end
+end