Allow users to opt out of automatic title rendering

Figured it'd be nice to allow more design flexibilty and let users opt-out
of automatic title rendering. In order to maintain backwards compatibility
the migration adds the column, defaults it to yes, and then goes back through
all existing spree_pages rows and marks the boolean as true.
Validate slugs are unique
Merge branch 'option_to_not_render_title' into 1-2-stable