refact(ci): extract reusable workflow

[gunar]

This PR DRYes the CI job that builds the React App. Similar code was being repeated in pull-request.yml, push-main.yml, and push-production.yml, and according to the rule of three it's time to refactor it.

The refactoring was not the sole goal though. This is a step towards #1442. In order to roll back a failed deployment, we have to actually roll forward. This means re-build (or possibly pull from cache) the frontend and any other dependencies. So really this PR came into existence to avoid adding yet another place (it'd be the 4th place) where CI builds the react app.

Rolling back failed deployments (#1442) will unblock scheduled production deployments (#1409).

[github-actions]

Pizza

Deployed 836fd48 to https://1496.planx.pizza.

Useful links:

• Editor
• Storybook
• Hasura
• API
• ShareDB

[github-actions]

Lighthouse

	buckinhamshire/FOIYNPP	testing/canary
performance	0.24 - 0.25 = -0.01	0.31 - 0.34 = -0.03
accessibility	1 - 1 = 0.00	1 - 1 = 0.00
best-practices	1 - 0.92 = 0.08	0.83 - 0.83 = 0.00
seo	0.88 - 0.89 = -0.01	0.89 - 0.92 = -0.03
pwa	0.7 - 0.7 = 0.00	0.7 - 0.7 = 0.00

[DafyddLlyr]

Closed in favour of this Trello ticket as this is now stale and de-prioritised.