Added skeletal activity block

blocks/activity/style.css

@@ -0,0 +1,11 @@
+/**
+ * The following styles get applied both on the front of your site and in the editor.
+ *
+ * Replace them with your own styles or remove the file completely.
+ */
+
+.wp-block-wp-strava-activity {
+	background-color: #000;
+	color: #fff;
+	padding: 2px;
+}