Deprecate block listings in favour of narrow

[waitingallday]

Expected behaviour

The narrow variant of block listings were intended as a replacement, but implemented separately to maintain compatibility with older sites. The narrow styles should be the only type of block listing.

Actual behaviour

There are two different implementations of the same component.

Proposed solution

The narrow variant inherits some style from the original block listing, so we'll need to merge the two implementations together. After this piece of work, the original block listing will be replaced with the narrow variant styles. This is a breaking change.