Prevent creation of redirect with duplicate "From" value

[johnbillion]

If a user creates a redirect with a "From" value that matches an existing redirect:

• The new redirect doesn't work because the existing one takes preference
• The post_name of the new redirect gets -2 appended, which means it will still not work even if the original is deleted because its hash won't match

The UI should show an error if you attempt to add or edit a redirect with a "From" value that matches and existing one, and suggest editing the original instead.