use the "hidden" flag to control whether a tag is selectable in Composer UI (and others?)

Tags have a boolean property called "hidden" that can be set in the TagManager UI, but has no effect elsewhere. It seems to be a legacy property from r2. see [#6009](https://github.com/guardian/flexible-content/issues/6009)

We want to re-purpose this flag to prevent "hidden" tags appearing in the UI so users cannot attach them to content. This will mostly be relevant for the obsolete commissioning tags that need to exist, but not be added to nee content.

Currently, the property is exposed in the internal tag manager API only, but not the tag manager hypermedia API consumed by workflow-frontend, composer and media-atom maker - the likely first step would be to add the "hidden" property to the model exposed on the hypermedia API

Start by tracing the Tag Manager hypermedia API model and compare it with the internal Tag Manager API, then check how workflow-frontend, Composer, and media-atom maker consume tags. The work is done when the hidden property is exposed through the hypermedia API and hidden tags no longer appear as selectable tags in the relevant UIs.

Written by the indexing model from the issue text.