Add optional property to suppress loading of BS3.

Jenkins bundled Bootstrap 3 grid uses untypical 24 columns
and is not compatible with new JS frameworks that are based
on BS4 and a 12 column grid.

Workaround for https://issues.jenkins-ci.org/browse/JENKINS-61326

core/src/main/resources/lib/layout/layout.jelly

@@ -58,6 +58,10 @@ THE SOFTWARE.
     <st:attribute name="type" use="optional">
       Available values: two-column (by default), one-column (full-width size) or full-screen (since 2.53).
     </st:attribute>
+    <st:attribute name="nogrid" use="optional">
+      Do not include Bootstrap 3 grid. When a plugin wants to use a more recent version of Bootstrap then
+      the bundled grid will cause conflicts.
+    </st:attribute>
   </st:documentation>
 <l:view contentType="text/html;charset=UTF-8">
 <st:setHeader name="Expires" value="0" />
@@ -107,7 +111,9 @@ THE SOFTWARE.
 
     <link rel="stylesheet" href="${resURL}/jsbundles/base-styles-v2.css" type="text/css" />
     <link rel="stylesheet" href="${resURL}/css/color.css" type="text/css" />
-    <link rel="stylesheet" href="${resURL}/css/responsive-grid.css" type="text/css" />
+    <j:if test="${attrs.nogrid==null or attrs.nogrid.equals(false)}">
+      <link rel="stylesheet" href="${resURL}/css/responsive-grid.css" type="text/css" />
+    </j:if>
     <j:if test="${attrs.css!=null}">
       <link rel="stylesheet" href="${resURL}${attrs.css}" type="text/css" />
     </j:if>