It's hard to build a pretty or compliant form without the Design
System. This commit (based on what's in Content Publisher) quickly
hacks a layout we can use.

Adds an `/admin/:document_type_slug` path, which reads the config
from the associated schema JSON file, and displays it in basic
form inputs.

Clicking "Generate schema" generates a JSON string which the user
can copy and paste. In theory, this makes requesting edits to
finders easier, as users can see the existing configuration, make
tweaks, and then give us the tweaked configuration in a ready to
use format. It also paves the way for self-serve in future, where
we may want to automatically republish finders after edits.

We can still lean on the "generate a big blob of JSON we can copy
and paste", while splitting the user experience up into more
manageable chunks.

For local dev only

Taking the same CSS and dependency as used in Travel Advice
Publisher.

The linked form currently goes to Tech 2nd Line, but we will
have it go to the publishing queue. See:
https://govuk.zendesk.com/agent/tickets/5930911