feat: Add CSAT response APIs (#2503)

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

db/migrate/20210623155413_create_csat_survey_responses.rb

@@ -0,0 +1,15 @@
+class CreateCsatSurveyResponses < ActiveRecord::Migration[6.0]
+  def change
+    create_table :csat_survey_responses do |t|
+      t.references :account, null: false, foreign_key: true
+      t.references :conversation, null: false, foreign_key: true
+      t.references :message, null: false, foreign_key: true, index: { unique: true }
+      t.integer :rating, null: false
+      t.text :feedback_message
+      t.references :contact, null: false, foreign_key: true
+      t.references :assigned_agent, foreign_key: { to_table: :users }
+
+      t.timestamps
+    end
+  end
+end