Spike: Evaluate server-side rendering (SSR)

[umpox]

For optimum performance, we should investigate SSR for the Sourcegraph web app. This will let us fetch data earlier and serve content to users quicker.

This issue should be used to evaluate different routes of SSR, from generating static pages, to simply prefetching data and inlining in our HTML.

It might be suitable to use this issue as a platform to create a new RFC.

This RFC should address:

1. Why SSR will benefit us
2. What is stopping us from using SSR
3. Proposed solution and migration plan

Previous Slack discussion: https://sourcegraph.slack.com/archives/C01LTKUHRL3/p1630481475006900

[github-actions]

Heads up @umpox @valerybugakov @5h1rU @pdubroy @taylorsperry - the "team/frontend-platform" label was applied to this issue.