[ZH][74718246] Accept hook object for Github#create_hook!

Pivotal Story: https://www.pivotaltracker.com/story/show/74718246
[ZH][74718246] Add hooks lookups and refactor shared behavior

Pivotal Story: https://www.pivotaltracker.com/story/show/74718246
[ZH][74718246] Improve interactor idempotence & minor view improvement

Pivotal Story: https://www.pivotaltracker.com/story/show/74718246

- Add wrapper method that only executes the block if condition is met
(condition defaults to checking for Rails.env.production?). Otherwise,
  we raise specific exception because these calls are destructive
  actions.

- Improves the layout of forms