step-style: add <a><paper-button> styling

All this time we've had the following "download" paper-button structure
on the official website at g.co/codelabs:

    <a href="..."><paper-button>download</paper-button></a>

Note that this is in accordance with the Polymer 1.0 recommendation
in https://elements.polymer-project.org/bower_components/paper-button/demo/index.html

Unfortunately, this is not how codelab-components were styled all this
time. A demo codelab has the inverse structure:

    <paper-button><a href="...">download</a></paper-button>

Here's where it's used in codelab-components repo:
https://github.com/googlecodelabs/codelab-components/blob/7397f708d210dfbb7348084033a32ba070ec4df4/demo/codelab.html#L74

As a consequence, the styling Will provided in a recent change in
f3e63d5
used the latter structure. We never noticed the difference because
before the styling refresh, the download button background and link
colors were very close.

step-style.html

@@ -194,6 +194,13 @@
         color: inherit !important;
       }
 
+      .instructions ::content a paper-button {
+        /* The text-decoration styling doesn't apply to inline blocks.
+           We use this trick to remove underline when paper-button
+           is wrapped in the anchor element. */
+        display: inline-block;
+      }
+
       .instructions ::content paper-button.colored {
         background-color: var(--google-material-green-600);
       }