Fix 'Save & Convert to Draft & Next' button

The javascript was not working as it was clobbered by the
double_click_protection.js. However it's not needed, we just need to set
the button name.

https://www.pivotaltracker.com/story/show/54183504
Merge pull request #744 from alphagov/54183504-fix-save-and-convert-to-draft-and-next

https://www.pivotaltracker.com/story/show/54183504

Fix 'Save & Convert to Draft & Next' button