Configure custom error page when provisioning domain

[davemcorwin]

User Story

As a user with a single page application (SPA) I want 404s to return ./index.html

Background (Optional)

cloud.gov's external domain broker supports specifying custom error pages for Cloudfront so when we provision the domain, we can provide this option when specified.

We also have some that currently have this enabled, might be worthwhile to investigate and port over the data.

We may need to consider adding update capability to the domain management so we can change this on an existing domain if necessary.

Acceptance Criteria

• [ ] an admin can set "SPA routing" when creating a domain
• [ ] Change made live via deploy federalist admin.

Level of effort - medium

Implementation outline (if higher than "low" effort):

• [ ] add a boolean column to represent SPA routing for a domain
• [ ] add this option to the UI/form flow
• [ ] When SPA Routing is true, set the equivalent of "error_responses": { "404": "/whatever/the/path/is/index.html" } when provisioning the domain