Port the build_site script

Rewrite the build_site rake task in Go or bash.

This script should accomplish the following:

• Clone product repositories
  • On CI, if the repository directories already exist, remove them
  • Locally, if the repository directories already exist, skip them
• Allow for fetching a specific Git branch and refspec

Skip generate_feature_flags in the rake file for now; this will be included in a separate issue.