Replace hardcoded Coveo api keys with a call to Netlify function

[sean-breen]

Proposed changes

Previously the values of each Coveo API key were hardcoded into the hugo theme code in layouts/partial/scripts.html which would pass the appropriate key value to the js.Build command. This would result in the api key being embedded in the minified javascript code, and observable via the inspection tools in a web browser.

This change removes those hardcoded values and replaces them with an API call to a Netlify function which will return the value of the COVEO_SEARCH_TOKEN variable from the Netlify site's environment variables.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[ADubhlaoich]

Approving for the intent of the effort and to ensure any minimum amount of approvals for merges is reached, but I defer to @nginx-jack to decide when the PR is ready to actually merge.

[nginx-jack]

Good to merge once it's validated on a dev environment in netlify 👍