Redo slider output and logic.

- Remove hardcoded 8 post limit for site features (fixes #133)
- Allow links in slider caption (fixes #100)
- Allow HTML in slider caption (fixes #115)
- Change the 'no slider' image because it conflicts with the caption
  added in commit ff69de6
- Adjust 'no slider' text to specify either 'Site Features' or 'Post
  Category' (see commit ff69de6)
- Remove redundant code