Manage cookbook URLs

* Rename 'external_url' to 'source_url' on Cookbook.
* Add 'issue_url' attribute to Cookbook.
* Create form and JS for handling ajax:success and ajax:failure for the form

Next steps:

* create the update action for cookbooks
* make sure the UI is good to go

Notes:

* for some reason the [x] on the alert box is not working, not sure why