Code Insights: Add Caution/suggest copy to the all repos creation form mode

Created by: vovakulikov

Closes https://github.com/sourcegraph/sourcegraph/issues/23260 Closes https://github.com/sourcegraph/sourcegraph/issues/23655

Context

This PR adds

• Some cautious description to the all-over repositories form control.
• Special mock and description messages to the live preview chart if all-repos mode has been turned on

You can find Figma designs here https://www.figma.com/file/wXKTspDmgTrPPumahHQTXQ/Caution-suggest-to-keep-insights-500-repos-23655?node-id=0%3A1

How to test

Since this all-repos mode is still under the feature flag experimentalFeatures.codeInsightsAllRepos You have to turn it on in your personal or org settings. Then you will see all-repos checkbox at the creation insight page /insights/create/search