+++ This bug was initially created as a clone of Bug #1368878 +++ Changes since last update: #8462 Removes last UMDs from the modules. #8463 Allow specifying the `PDFJS_NEXT` build flag via an environment variable when running the various `gulp` commands #8396 Adds streams-lib polyfill and exports ReadableStream from shared/util. #8394 Wraps mozL10n to async calls; splits firefox and generic l10n libs. #8466 Add a stub `createL10n` to `DefaultExternalServices` in app.js #8470 Fix inconsistent spacing and trailing commas in objects in `extensions/` files, so we can enable the `comma-dangle` and `object-curly-spacing` ESLint rules later on #8469 Fix inconsistent spacing and trailing commas in objects in `web/` files, so we can enable the `comma-dangle` and `object-curly-spacing` ESLint rules later on #8477 Fix inconsistent spacing and trailing commas in objects in `test/` files, so we can enable the `comma-dangle` and `object-curly-spacing` ESLint rules later on #8475 Fix inconsistent spacing and trailing commas in objects in remaining `src/` files, so we can enable the `comma-dangle` and `object-curly-spacing` ESLint rules later on #8474 Fix inconsistent spacing and trailing commas in objects in `src/core/` files, so we can enable the `comma-dangle` and `object-curly-spacing` ESLint rules later on #8478 Fix the remaining cases of inconsistent spacing and trailing commas in objects, and enable the `comma-dangle` and `object-curly-spacing` ESLint rules #8482 Convert the thumbnail view/viewer to ES6 syntax

Note that I had to re-apply the changes from bug 1353029, some of which required a bit of rebasing.

Pushed by ryanvm@gmail.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/08ba2d5f6d3b Update pdf.js to version 1.8.423. r=bdahl