Set custom key date values via the API

You can now set an agency-configured custom key date on an application with a
single request:

PUT /api/v1/applications/:application_id/custom_key_dates/:slug

Send { "data": { "attributes": { "date": "YYYY-MM-DD" } } } to set or update
the value, or { "data": { "attributes": { "date": null } } } to clear it. The
request is idempotent — you do not need to know whether a value already exists.
The response is 201 when a value is created and 200 when an existing value
is updated or cleared. A 404 is returned when the key date slug is not enabled
for the application's agency.