Fix #44 Remove unused data-used_on code
Hide remove button when used by page

But it won't be hidden when only used by another template.
I don't think we can easily determine if it's used by another
template, so deletions can still happen to partials by mistake if user
thinks it is safe to delete just because the button is there.
Merge pull request #45 from standout/safer-delete-button-for-page-templates

Even safer delete button for page templates