Review `app.bundle.js` for code-splitting optimizations

[umpox]

Similar to https://github.com/sourcegraph/sourcegraph/issues/26348 but higher priority as it is the bundle that we load on every page.

More context: https://sourcegraph.slack.com/archives/C014ZCKMCAV/p1634827138005800

When disabling Webpack concatenateModules

[github-actions[bot]]

Heads up @umpox @valerybugakov @5h1ru @pdubroy @taylorsperry - the "team/frontend-platform" label was applied to this issue.

[umpox]

Potential savings (size measured in gzip):

Dependencies that should potentially be lazy-loaded

• highlightjs: 34kb
• htmlparser2: 23kb
• marked: 15kb
• react-grid-layout: 10kb (insights)
• visx: 20kb (insights)
• react-draggable: 7kb (insights)
• d3/* related dependencies: 20kb (insights)

Potential savings: 129kb

Other code that should potentially be lazy-loaded

There's quite a bit here, in my view anything that isn't part of the nav bar or 'core UI' should be lazy-loaded. Few notable files: ~main.worker.ts: 86kb.~ -> Fixed by https://github.com/sourcegraph/sourcegraph/pull/26610 Folder: src/enterprise/insights: 17kb Folder: src/site-admin: 5kb

Potential savings: 108kb

Notes:

• Bundle analyzer doesn't support Brotli, so couldn't quickly get comparisons on savings here

Final savings

Potential savings: 237kb -> ~33% decrease in app.bundle.js bundle size