XWIKI-22800: Message boxes content doesn't extend to the right of the box (#3811)

* Cleaned up selectors. With the new improved DOM, there's no need for selectors specific to CKEditor
* Added `flex-grow` on the content+title+image element. This way, whatever is in it, it will use all the available space in the macro.
* Improved code structure

(cherry picked from commit e5af673a)

xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/less/messages.less

@@ -44,15 +44,15 @@ div.successmessage, div.errormessage, div.warningmessage, div.infomessage {
     max-width: unset;
   }
 
-  & > div > .box-title,
-  & > div > .xwiki-metadata-container[data-xwiki-parameter-name="title"] {
-    font-weight: bold;
-  }
-
-  // Main message content
-  & > div > p,
-  & > div > [data-cke-display-name="$content"] {
+  // Main message block
+  & > div {
     margin: 0;
+    flex-grow: 1;
+    
+    // By default the title is bold.
+    & > .box-title {
+      font-weight: bold;
+    }
   }
 }