general: provide support for a sidebar (#22440)

gadjo/static/css/gadjo.scss

@@ -285,14 +285,20 @@ div.errornotice {
 
 /* main content */
 
+#main {
+	display: -ms-flexbox;
+	display: flex;
+}
+
 div#main-content {
+	-ms-flex: 0 1 auto;
+	flex: 0 1 auto;
 	width: 100%;
 	margin-left: 0;
 	padding: 1em 6px 0 16px;
 	border: 1px solid #ccc;
 	border-width: 0 1px 1px 0;
 	background: white;
-	float: left;
 	position: relative;
 	overflow-x: auto;
 	overflow-y: hidden;
@@ -302,6 +308,18 @@ div#main-content {
 	}
 }
 
+#sidebar {
+	-ms-flex: 0 1 auto;
+	flex: 0 1 auto;
+	margin-left: 1rem;
+	margin-right: 1rem;
+	width: 30rem;
+	max-width: 30rem;
+	color: #666;
+	position: relative;
+}
+
+
 div#main-content h2 {
 	font-weight: normal;
 }

gadjo/templates/gadjo/root.html

@@ -101,6 +101,9 @@
     {% block after-main-content %}
     {% endblock %}
 
+    {% block sidebar %}
+    {% endblock %}
+
   </div> <!-- #main -->