Add a utility class to remove margin top. (#4742)

blockbase/assets/ponyfill.css

@@ -244,6 +244,10 @@ p {
 	padding: 0;
 }
 
+.mt-0 {
+	margin-top: 0 !important;
+}
+
 /**
  * Elements
  * - Styles for basic HTML elemants

blockbase/sass/base/_utility.scss

@@ -11,4 +11,9 @@
 //Removes padding on Column block when it has a background color
 .has-background-no-padding.wp-block-columns.has-background {
 	padding: 0;
+}
+
+// For when we do not want the vertical block gap. Needed until we have per-block margin controls.
+.mt-0 {
+	margin-top: 0 !important;
 }