Reworking Message component

* Supporting URLs for icon
* Moving styles into top-level stylesheet for use in the template

src/components/__snapshots__/Message.test.js.snap

@@ -0,0 +1,28 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Message rendering 1`] = `
+<div
+  className="message"
+>
+  <h2>
+    Testing
+  </h2>
+  <p>
+    Message content
+  </p>
+</div>
+`;
+
+exports[`Message rendering with icon 1`] = `
+<div
+  className="message"
+>
+  <h2>
+    Sample icon SVG
+    Testing
+  </h2>
+  <p>
+    Message content
+  </p>
+</div>
+`;