Cookbook deprecation adjustments

:fork_and_knife:

This is a pull request to improve the cookbook deprecation.

Working on the QA items from: https://trello.com/c/6gyEzuW2

technical todo:

- [x] the logic in `CookbooksController#deprecate_search` should be moved into an instance method on Cookbook
- [x] display error when cookbook deprecation fails
- [x] do not allow submission of the deprecation form w/o the selection of a replacement

There are additional things that should be done to improve the cookbook deprecation, so I created a card for them: https://trello.com/c/mDFbDEhs This keeps this PR a little smaller and manageable.

This will raise a `KeyError` if the `q` param is missing since there's no default value set. Is that what we want?