Users can manage cookbook url data

:fork_and_knife: 

* 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

todo:

- [x] write feature spec
- [x] create the update action for cookbooks
- [x] make sure the UI is good to go
- [x] add url validation
- [x] hide buttons if urls are removed after save
- [x] show buttons if urls are added after save