load filepicker javascript asset only when needed

I created a new `yield :extra_js` area in application_layout which can be used to add extra javascript assets on demand in a specific view.

I used this new area in `app/views/proofs/new.html.erb` and `app/views/proofs/edit.html.erb` to load filepicker js asset.

I know it doesn't speed the app up a lot, but it helps.