Fix typo.
Fix permitted params bug when editing a promo item with links.
Relax requirements for promotional feature items.

- 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.
Make duplicate fields work with double nesting

On promotional features the duplicated fields are part of two array
blocks. This means there was a second 'array like' string in the field
name. Previously this incremented the first one, incrementing the second
one is more appropriate.
Merge pull request #1427 from alphagov/reduce-promo-feature-requirements

Relax requirements for promotional feature items