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

[silent1mezzo]

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.