Clean up participant hash/slug methods (tech debt)

Summary

We use a few different methods for producing the participant hash and slug (hash-name) in the Responses, Consent and Contact Participants pages, and in the response downloads. This makes it difficult to ensure that hashes/slugs match across the different views. We should clean this up so that we have a central method that can be imported and used in all cases.

• Contact Participants page: participant_hash/participant_slug
• Individual Responses page: child_id_slug, parent slug (created via JS)

There may be more places that use the slug. It can be created as a hash-nickname string concatenation, rather than via a helper method. Ideally we would always use the same helper method for this in the view classes and pass the value into templates etc. (rather than creating the slug 'on-the-fly' inside templates or JS).

Start by locating participant_hash and participant_slug on the Contact Participants page, child_id_slug and the parent slug on the Individual Responses page, and slug construction in response downloads and JavaScript. Trace other hash-nickname concatenations across the view classes, templates, and JS, then confirm that all listed views and downloads use one shared helper and produce matching values.