feat: Add assignee last seen to conversations (#3069)

- Adds assignee last seen attribute to conversations

db/migrate/20210922082754_add_assignee_last_seen.rb

@@ -0,0 +1,5 @@
+class AddAssigneeLastSeen < ActiveRecord::Migration[6.1]
+  def change
+    add_column :conversations, :assignee_last_seen_at, :datetime
+  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.define(version: 2021_09_16_060144) do
+ActiveRecord::Schema.define(version: 2021_09_22_082754) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_stat_statements"
@@ -296,6 +296,7 @@ ActiveRecord::Schema.define(version: 2021_09_16_060144) do
     t.bigint "campaign_id"
     t.datetime "snoozed_until"
     t.jsonb "custom_attributes", default: {}
+    t.datetime "assignee_last_seen_at"
     t.index ["account_id", "display_id"], name: "index_conversations_on_account_id_and_display_id", unique: true
     t.index ["account_id"], name: "index_conversations_on_account_id"
     t.index ["assignee_id", "account_id"], name: "index_conversations_on_assignee_id_and_account_id"