add feature#edit interface elements specified in "updating features" user story

[del82]

https://github.com/del82/ezra/wiki/User-roles#updating-features-101

• [x] feature#edit: contains all of the fields that the feature#new screen does, as well as the instructions on the feature#new screen
• [x] feature#edit: contains a "save" and a "cancel" button. When the user clicks the "save" button, the feature is updated and she is returned to the feature index. A flash message appears confirming that the feature updates have been saved. When the user clicks the "cancel" button, the feature is not update and she is returned to the feature index. A flash message appears confirming that the changes were not saved.

[parryc]

Oops, didn't read through the entirety of feature#new. I'll close this once I finish that.