Draft: Split company build to dedicated job

NOTE: Closing this MR without action. See notes in #8302 (closed) for explanation of reduced scope

What does this MR do?

Split build 8/9 ("company") out of PartialBuild to separate dedicated jobs:

• This first dedicated job will introduce extensions/partial_site_build. This will use PARTIAL_SITE_BUILD_SOURCE_PATHS, which is a comma-delimited list of paths (regexes may be used) under source (Middleman "resource.destination_path") which will be built. The extension will be very similar to the manipulate_resource_list in extensions/monorepo.rb.
• Update scripts/deploy to use PARTIAL_SITE_BUILD_PATHS_TO_DEPLOY. This is a comma-separated list of top-level paths to be partially deployed. Will need to introduce a loop with a separate rsync command for each path.

Tasks

• Write the extension
• Write ci jobs to use the extension (use handbook jobs as a template)
• Remove reference to company from PartialBuild
• Verify that exactly the same files are built and published in the artifact as current master code with PartialBuild.
• Test with local dev env
• Test with local build
• Test in review app
• Test in production

Related issues

Relates #8302 (closed)