DEV: use application-layout for crawler Chrome-Lighthouse (PR #15592)

Exclude user-agent Chrome-Lighthouse from crawlers and show it the real application view.

This enables Lightouse in Chrome DevTools and PageSpeed Insights to measure the performance of Discourse like viewed by an user. Until now Lighthouse measured the performance for an user-device which is rendering the crawler-layout - this never happens in reality.

GitHub

This pull request has been mentioned on Discourse Meta. There might be relevant details there:

Closed forever until Google “run[s] those lighthose performance tests on a recent iPhone model (even an iPhone 8 or later)”.

See Real performance data for Lighthouse - #2 by codinghorror - dev - Discourse Meta