Design: Make the sidebar minimum 12 em wide.

Change-Id: I4752c7b92df2ff835b12d0b7e00cb29e1128e0f5
diff --git a/src/main/resources/META-INF/resources/cms2/base.css b/src/main/resources/META-INF/resources/cms2/base.css
index d635110..8ff1836 100644
--- a/src/main/resources/META-INF/resources/cms2/base.css
+++ b/src/main/resources/META-INF/resources/cms2/base.css
@@ -74,7 +74,7 @@
 
 @media (min-width: 30em) {
   body {
-    grid-template-columns: auto 3fr;
+    grid-template-columns: minmax(12em, auto) 3fr;
     grid-template-rows: auto 1fr auto;
     grid-gap: 0;