Consolidate slider metaboxes.

- Rename 'Slider Caption Text' metabox to 'Slide Options'.
- Remove the 'Custom URL/Permalink' metabox and move the 'Custom URL'
  to the 'Slide Options' metabox.
- Localize metabox labels and descriptions.
- Use esc_url() when outputting slider custom URL.
Trim whitespace.
Relabel 'features' post type to be more specific for UX.

Previously, the 'features' post type used ambiguous labels like 'Site
Features' or 'Add New Feature' when the post type only currently supports
a homepage slider.

When the 'features' post type supports other things, we can relabel the
post type again when suitable.