feat(snackbar): Update to match latest design guidelines (#4166)

### Issues fixed

* Fixes #4005 (via new `mdc-snackbar-viewport-margin()` mixin)
* Fixes #3981
* Fixes #2916 (via new `MDCSnackbar:closing` and `MDCSnackbar:closed` events)
* Fixes #2628
* Fixes #1466 (via new `close()` method)
* Fixes #1398
* Fixes #1258
* Fixes #11 (the label now expands indefinitely to fit the text)
* Refs #2813

### Screenshots

![Baseline without action button](https://user-images.githubusercontent.com/409245/49956261-a080ca80-feb9-11e8-8287-b7e805344115.png)

![Baseline with action button](https://user-images.githubusercontent.com/409245/49956109-4a138c00-feb9-11e8-9213-3241fa86a1b8.png)

![Stacked layout](https://user-images.githubusercontent.com/409245/49956173-6a434b00-feb9-11e8-8a03-2e58ccc8f763.png)

BREAKING CHANGE: Snackbar's DOM and APIs have changed to match the latest design guidelines. See the Snackbar documentation for more information.

demos/index.html

@@ -172,14 +172,6 @@
             </span>
           </a>
 
-          <a href="snackbar.html" role="listitem" class="mdc-list-item">
-            <span class="demo-catalog-list-icon mdc-list-item__graphic"><img src="/images/ic_toast_24px.svg" /></span>
-            <span class="mdc-list-item__text">
-              <span class="mdc-list-item__primary-text">Snackbar</span>
-              <span class="mdc-list-item__secondary-text">Transient messages</span>
-            </span>
-          </a>
-
           <a href="tab-bar.html" role="listitem" class="mdc-list-item">
             <span class="demo-catalog-list-icon mdc-list-item__graphic"><img src="/images/ic_tabs_24px.svg" /></span>
             <span class="mdc-list-item__text">

package.json

@@ -237,6 +237,7 @@
     "mdc-select",
     "mdc-selection-control",
     "mdc-slider",
+    "mdc-snackbar",
     "mdc-switch",
     "mdc-tab",
     "mdc-tab-indicator",