bug 875349: Build JSON metadata for Documents at rendering time

* Also, adds `?expand` parameter to `$children` view for [bug 874311](https://bugzilla.mozilla.org/show_bug.cgi?id=874311). That expands the metadata for each page listed in the doc tree, but uses stale cache wherever possible.

Okay, I think this is ready for more eyes. Possibly even a merge. Mainly, this is an API change. Here are some example URLs to peek at, before and after editing docs:

https://developer-local.allizom.org/en-US/docs/Web/HTML$json
https://developer-local.allizom.org/en-US/docs/Web$children?expand
https://developer-local.allizom.org/en-US/docs/Web$children?expand

Mileage will vary depending on what docs you have locally. Basically, just stick `$json`, `$children`, and `$children?expand` after various wiki URLs and see what happens.