FIX: Migrate post_edit_time_limit to tl2_post_edit_time_limit (#8082)

If a user amended edit_time_limit keep the behavior as is, instead of introducing a potentially tighter time for tl2 edit time than what they had set in the past.

db/migrate/20190908234054_migrate_post_edit_time_limit.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+class MigratePostEditTimeLimit < ActiveRecord::Migration[5.2]
+  def up
+    execute <<~SQL
+    INSERT INTO site_settings (
+      name,
+      value,
+      data_type,
+      created_at,
+      updated_at
+    )
+      SELECT
+        'tl2_post_edit_time_limit',
+        value,
+        data_type,
+        CURRENT_TIMESTAMP,
+        CURRENT_TIMESTAMP
+      FROM site_settings
+      WHERE
+        name = 'post_edit_time_limit'
+    ON CONFLICT
+    DO NOTHING
+    SQL
+  end
+end