chore: Add index on conversations.uuid (#5179)

- We search for conversations by this attribute (e.g. in ReplyMailbox and CsatSurveyController) so it seems like we should have an index on it.

db/migrate/20220802193353_add_conversation_uuid_unique_index.rb

@@ -0,0 +1,5 @@
+class AddConversationUuidUniqueIndex < ActiveRecord::Migration[6.1]
+  def change
+    add_index :conversations, :uuid, unique: true
+  end
+end

db/schema.rb

@@ -413,6 +413,7 @@ ActiveRecord::Schema.define(version: 2022_08_09_104508) do
     t.index ["last_activity_at"], name: "index_conversations_on_last_activity_at"
     t.index ["status", "account_id"], name: "index_conversations_on_status_and_account_id"
     t.index ["team_id"], name: "index_conversations_on_team_id"
+    t.index ["uuid"], name: "index_conversations_on_uuid", unique: true
   end
 
   create_table "csat_survey_responses", force: :cascade do |t|