Dedupe list-entry shared attributes
Dedupe list-entry shared attributes

The `getSharedAttributes` method gathers all
attributes, and [DOM tokens](https://developer.mozilla.org/en-US/docs/Web/API/Element/classList)
in the `class` attribute not controlled by the
module for re-applying in the `render` method.

Because it gathered them for all items, many
duplicates ended up being added to new items.

Browser engines were tidying up the duplicates
but we shouldn't be adding them in the first
place.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin