Trigger preview builds and deploys from static_build

A big pull request since I had to refactor lots of things:

• Changed the way static_build is configured: now there are two groups, one for live and another for preview builds.
• Moved EventSubscriber from static_preview_gatsby_instant to static_build, and allow the use of more than one builder for preview.
• Added new --request-build and --request-deploy to static_export commands
• Added new --request-deploy to static_build commands
• Fix several bugs in ReleaseControllers