Add headless embeddable mode

[obulat]

Fixes #63
This PR adds headless embeddable mode to the application so that we can embed it into an iframe.

Embedded mode

The default mode embedded mode has smaller header without logo, and no footer. You can also explicitly set the embedded mode by using a embedded=true parameter in the URL http://localhost:8443/?embedded=true . A router middleware checks for this parameter, and saves any changes to the vuex store.

---

Standalone mode

The standalone mode, with a larger header, logo and footer, can be activated through the embedded=false parameter in the URL: http://localhost:8443/?embedded=false when running locally.

[zackkrida]

I like the middleware + vuex approach, works well with the existing data model.

Can you switch the h1 in the hero in embedded mode to just say "Openverse"? I feel like the existing text is redundant anyway 😆

[dhruvkb]

We discussed this on Slack but I just wanted to formalize it as a comment. When running in embedded mode, a message should be posted to the window.parent containing the page height so that the embedder can set the proper height on the iframe. This message should be posted on first load, and subsequently after every resize of the viewport width.

A simple object containing a height property should be sufficient for the message argument in postMessage.

Example: resize_test.html at WordPress/wordpress.org#31