allow users to edit story URLs

We see a steady stream of typos, finding better links, or people getting
suprised by the tweaks in Story#url=. Edits go to the modlog, so the risks of
abusive URL replacement seems pretty low.

app/models/story.rb

@@ -516,7 +516,7 @@ def is_editable_by_user?(user)
       if is_moderated?
         false
       else
-        (Time.current.to_i - created_at.to_i < (60 * MAX_EDIT_MINS))
+        created_at.after?(MAX_EDIT_MINS.minutes.ago)
       end
     else
       false
@@ -872,6 +872,8 @@ def url=(u)
   def url_is_editable_by_user?(user)
     if new_record?
       true
+    elsif !is_moderated? && created_at.after?(MAX_EDIT_MINS.minutes.ago)
+      true
     else
       user&.is_moderator?
     end