search history: add feature flag to disable

Created by: limitedmage

Context on Slack

This adds the feature flag search-input-hide-history that will hide the search history shown when focusing on the search input.

Test plan

Use the Site Admin page to turn the feature flag on and off and check that the search history is hidden when the feature flag is on, and shown when the feature flag is off or non-existant.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.