Performance: Streamline `app.bundle.js` size

[umpox]

Description

Goal: Reduce code required to be downloaded before every page can be shown.

Reason:

• This bundle is loaded on every page, and required before we can render anything in the Sourcegraph application.
• It may be high value, low effort. From an initial investigation, there is lots of areas where we’ve accidentally included things, or broken lazy loading.

Related PR: https://github.com/sourcegraph/sourcegraph/pull/38817

A/C

• Lower app.bundle.js size
• Update our bundlesize config to enforce this and ensure we don't regress in future.

[sourcegraph-bot-2]

Heads up @taylorsperry @jasongornall - the "team/frontend-platform" label was applied to this issue.