build: fix routing playground example using external resource (#28490)

Currently the "routing" playground example fails the e2e tests
because it tries to load the OpenSans font using an external
HTTP request. External http requests are not allowed (unless
explicitly enabled) within Bazel in order to ensure that
all targets are built and tested in a hermetic way.

In order to work around this issue in a Bazel idiomatic way,
we just vendor the fonts in the "third_party" folder. Note
that we can technically also enable internet for the RBE
host platform, but it's not a best practice for hermeticity.

The following syntax would allow us to enable internet for
RBE (stated here for tracking)

```
properties: {
  name: "dockerNetwork"
  value: "standard"
}
```

PR Close #28490

modules/playground/src/routing/BUILD.bazel

@@ -24,6 +24,7 @@ ts_devserver(
         # This is temporarily needed because Angular imports from "rxjs/operators/index", while
         # there is only one RxJS UMD bundle that re-exports everything at the root.
         "//modules/playground:systemjs-rxjs-operators.js",
+        "//third_party/fonts.google.com/open-sans",
         "@ngdeps//node_modules/rxjs:bundles/rxjs.umd.js",
         "@ngdeps//node_modules/tslib:tslib.js",
     ] + glob(["**/*.css"]),

modules/playground/src/routing/css/gumby.css

@@ -1,15 +1,7 @@
-/**
- * Angular note: local copy downloaded from https://cdnjs.cloudflare.com/ajax/libs/gumby/2.6.0/css/gumby.css
- * local modification:
- *    - forced google fonts to be fetched via http in order to avoid SSL error
- *      https://fonts.googleapis.com/css?family=Open+Sans:400,300,600,700 - Failed to load resource: net::ERR_SSL_PROTOCOL_ERROR
- *      it's unclear why the CI tries to fetch the font via SSL when the base url is http
- *      example broken build: https://travis-ci.org/angular/angular/jobs/204456086#L2355
- *
- */
-
 @charset "UTF-8";
-@import url(https://fonts.googleapis.com/css?family=Open+Sans:400,300,600,700);
+
+@import url("/angular/third_party/fonts.google.com/open-sans/open-sans.css");
+
 /**
 * Gumby Framework
 * ---------------