chore: API improvements (#3637)

- Unique validations for Inbox members and Team member objects
- Move notification processing to Async

db/migrate/20211221125545_add_unique_index_on_inbox_members.rb

@@ -0,0 +1,19 @@
+# ref: https://dev.to/nodefiend/rails-migration-adding-a-unique-index-and-deleting-duplicates-5cde
+
+class AddUniqueIndexOnInboxMembers < ActiveRecord::Migration[6.1]
+  def up
+    # partioning the duplicate records and then removing where more than one row is found
+    ActiveRecord::Base.connection.execute('
+      DELETE FROM inbox_members WHERE id IN (SELECT id from (
+        SELECT id, user_id, inbox_id, ROW_NUMBER() OVER w AS rnum FROM inbox_members WINDOW w AS (
+          PARTITION BY inbox_id, user_id ORDER BY id
+        )
+      ) t WHERE t.rnum > 1)
+    ')
+    add_index :inbox_members, [:inbox_id, :user_id], unique: true
+  end
+
+  def down
+    remove_index :inbox_members, [:inbox_id, :user_id]
+  end
+end