manage trim links

* Add, change, remove, undo a trim link
* Made the association singular as there is only one trim link associated with a question
* Removed some superfluous JavaScript
* Add deleted flag to support undo feature
* Added supporting feature specs (wuhoo!)

Additional work done:

* make delete secondary button style
* cancel link when file selected for upload
* styling for undo and cancel to look like a link
* wording change for 'Trim link deleted'
* Ensure errors are displayed when link invalid and ensure form fields are not lost and previous link is restored

Not doing:

* go to previous tab selected - as we're possibly changing this it's not worth the effort