Relax requirements for promotional feature items

- [x] Fix permitted params bug (@elliotcm)
- [x] Fix link form duplication bug (@edds)
- [x] Do the actual ticket (@elliotcm)

Changes:
- Remove requirement for an image
- Remove cap on links within an item

This is primarily driven by need for a link-heavy item without an image for the Civil Service page.

We can add restrictions back in if it begins to be abused.