block use from changing url of source

Sorry, I should have told you there is already a mechanism for this:
```
   updates.for(:content)  # boolean.  tells you whether content is updated
   content_without_tracking  # gives you previous content
```

All of this functionality is in attribute_tracking.rb and is initialized for the content field in lib/card.rb with the line "tracks :content".

We actually used to make much greater use of this code, but now for other fields (like name, type_id, etc), we're using Rails' "Dirty" handling.  (see http://api.rubyonrails.org/classes/ActiveModel/Dirty.html)

We can't use that with the content field yet, because content is not actually a field on the cards table (it's only on card_revisions), but we may be reorganizing the data soon to change that.

-e