Allow site owner to configure publish directory

[davemcorwin]

User Story

Add configuration option for site owners to configure a directory other than _site to be published.

Background (Optional)

This would be super helpful for sites where the user doesn't have good control over the output directory of the build (looking at YOU Gatsby!). We currently rely on symlinks to simulate this, but they can be problematic in certain situations.

Could be configured in UI or in federalist.json.

Acceptance Criteria

• [ ] Site owner can configure the directory to publish to S3 (something other than _site)
• [ ] The directory to publish must be restricted to those that are safe (not /root or whatever)
• [ ] Change made live via <TODO>.

Level of effort - medium

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

• [ ] <TODO>