chore: Add sla policy association to conversation (#7360)

Adds the sla policy association to the conversation

Fixes: https://linear.app/chatwoot/issue/CW-1615/applying-an-sla-to-the-conversation

db/migrate/20230620132319_add_sla_policy_to_conversations.rb

@@ -0,0 +1,5 @@
+class AddSlaPolicyToConversations < ActiveRecord::Migration[7.0]
+  def change
+    add_column :conversations, :sla_policy_id, :bigint
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2023_06_12_103936) do
+ActiveRecord::Schema[7.0].define(version: 2023_06_20_132319) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"
@@ -448,6 +448,7 @@ ActiveRecord::Schema[7.0].define(version: 2023_06_12_103936) do
     t.datetime "assignee_last_seen_at", precision: nil
     t.datetime "first_reply_created_at", precision: nil
     t.integer "priority"
+    t.bigint "sla_policy_id"
     t.index ["account_id", "display_id"], name: "index_conversations_on_account_id_and_display_id", unique: true
     t.index ["account_id", "id"], name: "index_conversations_on_id_and_account_id"
     t.index ["account_id", "inbox_id", "status", "assignee_id"], name: "conv_acid_inbid_stat_asgnid_idx"