Update URL structure for promises to use a slug instead of ID

**Is your feature request related to a problem? Please describe.**
To improve SEO and readability of links we could update the URL structure from `/:department/promises/:id` to `/:department/promises/:slug`

**Describe the solution you'd like**
There's a few different ways to do this.

1. slugify the `concise_title`. This approach is the cleanest from a URL structure but requires updates in both this repo and the API.
2. slugify the `concise_title` and concat it onto the ID (e.g. `/:department/promises/:id-slug`. This is more involved on the FE as you have to strip the slug from the ID but doesn't require a change in the API.