Re-instate Google Maps script appending client-side

[jng]

Although the unified entry point work removed loading Google Maps script tags client-side, upon second thought: It is possible to "safely" load Google Maps on the client-side (don't document.write the script tags, rather document.createElement the required script tags just like we do for the widgets).

This way, we don't need to force a breaking migration change to the new unified entry point and existing users can still remain on the index.html entry point.

[jng]

In 3047:

Re-instate client-side Google Maps script loading. It slipped my mind when I removed the client-side loading that we can actually do this in a "safe" manner.

We just document.createElement() the script tag instead of document.write()'ing it. This is what we're doing for the widget scripts.

As part of this change, several synchronous XHR requests we were making have been converted to async as well.

Fixes #664

[jng]

In 3048:

Merged revision(s) 3047 from trunk:
Re-instate client-side Google Maps script loading. It slipped my mind when I removed the client-side loading that we can actually do this in a "safe" manner.

We just document.createElement() the script tag instead of document.write()'ing it. This is what we're doing for the widget scripts.

As part of this change, several synchronous XHR requests we were making have been converted to async as well.

Fixes #664
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