Modularise build script for various scenarios (new site, selected sites, all sites)

[natesheehan]

Use case

As a moderator
I want to rerun the build script for various scenarios (new site, selected sites, all sites)
So that the build script doesn't need to be edited every time we rerun

To do

• Enable build for a new site
• Enable build for selected sites
• Enable build for all sites
• Write tests and sanity checks for all scenarios
  Extention:
• Set up GitHub actions for selected sites and all sites.