Adds filepicker save link

I added a new helper method to ApplicationHelper, called filepicker_save_link. It works just like filepicker_save_button but generates a link instead. This functionality is supported by the Export Widget in the Filepicker API, and for the project I'm currently working on, it is better to have a link than a button styled as a link.

Specs and docs are in, yet they are pretty similar to filepicker_save_button.