Lighthouse: Implement historical monitoring

Created by: umpox

Description

We had weekly Lighthouse reports that would report to #dev-frontend on Slack. These would run 10 audits and then take the average result and post it.

These had some problems:

• Even with 10 audits, it was naturally quite flaky. This is an inherent issue with Lighthouse (https://github.com/GoogleChrome/lighthouse/issues/10657), and most performance monitoring tools
• We couldn't view historical reports, we used Googles temporary storage which only kept reports for a week or so. It wasn't easy to diff between weeks to view trends.

We discussed with Code Insights a potential solution of using the compute engine to show historical trends: https://sourcegraph.slack.com/archives/C01C3NCGD40/p1643030849067100?thread_ts=1643027572.066100&cid=C01C3NCGD40

Unfortunately this isn't currently possible, we need to check in with Code Insights to see if there's a timeline on this or move to a typical solution using Lighthouse server