Add nested ember route 'edit' (under ideas/idea)
Add an IdeaEdit template, to flesh out controller actions
Add comment about `model.foo` too explicit; via tutorial comments
Add (ember) IdeaEditController
Add 'edit' link to singular idea template
Remove unfortunate copy-pasta duplication from idea edit template
Add outlet to idea template (so edit template has somewhere to show up)
Set `isEditing` property on Idea controller whenever IdeaEdit route is activated.

Show/hide the content in the template as appropriate.
Add comments to clarify the usage of `isEditing` on the IdeaController
Merge branch 'ch17-editing-lead-pt2'