Router V6 Migration: add CompatRouter and update Link component

Created by: oleggromov

This is a roll-back of a rolled back original first PR in the router v5 --> v6 migration sequence.

See the original for description and context: https://github.com/sourcegraph/sourcegraph/pull/36285

What changed

ReferencesPanel, which crashed with the previous update because it was mistakenly wrapped in a <CompatRouter> too, is fixed.

For details, see slack threads:

• covering the first bug report and the following roll-back
• explanation of what's happened and why

Test plan

1. Pull the branch
2. Launch the app locally and make sure all main routes work
3. Check explicitly that the references panel works

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.