Grant ability to add variable number of candidate books when submitting recommendations

[jimchamp]

It was decided during design discussions that recommenders provide at least one comparable candidate book on submission of a recommendation. Currently the UI has two fields for candidates, and I believe that the back-end expects three candidates on submit (more details on this in issue #35). This poses problems for folks that want to include more than two candidate works in a recommendation, and may be confusing for people that only want to include one.

Proposed enhancements to solve these problems:

1. Display only one candidate field initially with an "Add" button.
2. When a work is added, it will appear below the field in a list of candidate books.
3. Each list item can be easily removed, just in case it was added in error.
4. Make back-end changes to allow for a variable amount of candidate works.

It would also be worthwhile to add front-end validation to ensure that at least one candidate is present on submission.