Add Algolia search

[Scharrels]

This patch adds an algolia search feature to the Trailblazer site.

To use this patch, the following things need to be done:

• Obtain an account at https://www.algolia.com/
• Generate an API key
• Configure the application keys in _config.yml
• Generate the search index (using the command in the readme)
• Regenerate the site
• ...
• Profit!

The search page uses the "Search by Algolia" logo, as I expect that Trailblazer can use an open source license.

The search page has its own customizable theme color, found in scss/components/_gems.scss.