Improve output when watching for changes

[kzar]

Commands such as npm run dev-chrome call something like make watch browser=chrome type=dev under the hood. The CLI output was a little
confusing, since it wasn't clear when the initial build had
finished. Let's improve that by keeping make silent until there are
changes, and displaying a clear "Build ready - Watching for changes"
message.

Reviewer: @sammacbeth

Steps to test this PR:

1. Run npm run dev-chrome and check the output makes sense.

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications