Make has_many nested records' Remove button uniform

Hello,
At the moment, has_many nested record form offer two ways to remove a record, depending on wether it's new (Remove button that also removes the record form from the page) or existing (simple destroy checkbox).
I propose in this pull request to render this remove feature uniform, by replacing the destroy checkbox by a Remove button similar to the one used on new records.
The destroy checkbox becomes a hidden input, which is set to true when the user clicks on Remove. The record's form is also hidden in the process.
Sortable still works.
I also updated rspec tests.

Thanks in advance for your review,

David