Rename meta css helper to metadata-list

The 'meta' class was too generic and was causing confilcts. I've left
the meta class in as well as it seems semanticlly useful and some tests
rely on it.
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 #1424 from alphagov/fix-meta-style

Rename meta css helper to metadata-list
Merge pull request #1427 from alphagov/reduce-promo-feature-requirements

Relax requirements for promotional feature items
Send organisation slugs to Panopticon

Sent when registering editions.