[Google Search] Set up basic search forms

Set up search form components and wire them up to Google search.

Steps:

• Set up a sandbox account on Google Cloud Platform (https://about.gitlab.com/handbook/infrastructure-standards/realms/sandbox/)
• Enable Custom Search API and create a scoped API key
• Add markup and configuration to use Google for search on the site (similar conditionals to Algolia vs Lunr -- Google should only be used in the GitLab production environment, not self-hosted installs or archived versions)
• Extend the existing Lunr forms to be able to use Google.
• Show results and result count on the /search page.

This work should be deployable/mergable without impacting the existing Algolia search.

Done when:

• Using an environment variable, we can create a build that uses Google instead of Algolia and run this in a review app or locally.
• We can enter a search query in basic search forms on the site and see results delivered from Google on a dedicated results page (no autocomplete, no modals for now).
• A search without results returns a "no results" message.
• Human-friendly error if we are unable to reach the Google API.
• Results page shows result count ("showing x-y of z results"), and we can see the first ten results (pagination comes later: #1376 (closed))