Submit input field when pressing Return (#1537)

Community-specific part of
#897. Users can now submit
a new hostname by pressing the “Return” key.

It seems that the [`Element.click()`
method](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/click)
is exactly what we need in terms of behaviour, and I thought it made
more sense to make the implementation so that it “delegates” to the
button click-handler, rather than repeating the internal logic of the
latter.

To me, the event handler would be expressive and straightforward enough
to be inlined.

I also added a brief explanation in the style guide, to capture our
intent.
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app/templates/custom-elements/change-hostname-dialog.html

@@ -159,6 +159,11 @@ <h3>Changing Hostname</h3>
           this.elements.changeAndRestart.addEventListener("click", () => {
             this._doChangeHostname();
           });
+          this.elements.hostnameInput.addEventListener("keydown", (evt) => {
+            if (evt.code === "Enter") {
+              this.elements.changeAndRestart.click();
+            }
+          });
           this.elements.cancelInitialization.addEventListener("click", () => {
             this.dispatchEvent(new DialogClosedEvent());
           });

app/templates/styleguide.html

@@ -256,6 +256,13 @@ <h2 class="section">Input</h2>
           <pre>autocapitalize="off"</pre>
           <pre>spellcheck="false"</pre>
         </li>
+        <li>
+          If there is a call-to-action button associated with an input field,
+          then pressing the “Enter”/“Return” key on the keyboard should trigger
+          the call-to-action. The behaviour should be the same as if the button
+          was clicked with the mouse; that implies that the call-to-action
+          shouldn’t trigger in case the button is in disabled state.
+        </li>
         <li>
           If the input value is required to adhere to a certain format, use the
           placeholder attribute to help the user intuitively grasp the expected