Add search to API site

[Krinkle]

We can use the same method as for the main site, using Algolia.

Tentatively:

• Add the API secret to Travis CI config as environment variable for the main branch.
• Add the Travis step (ref https://github.com/qunitjs/qunitjs.com/blob/f0e71d1a12/.travis.yml).
• Include its results in the search on the main site. See also How to display results from multiple index.

And as part of #1458, naturally, to also display the search bar on the API site itself.

[Krinkle]

Before	After