fix: set default content type as text for message (#3060)

Fixes #2974

db/migrate/20210923132659_set_content_type_text_for_the_old_messages.rb

@@ -0,0 +1,5 @@
+class SetContentTypeTextForTheOldMessages < ActiveRecord::Migration[6.1]
+  def change
+    change_column_null(:messages, :content_type, false, 0)
+  end
+end