Restore baseline width to .section-divider via flex-grow

Author: jacbn

src/scss/common/elements.scss

@@ -339,6 +339,7 @@ iframe.email-html {
   overflow: hidden;
   border-top: 1px solid rgba(0, 0, 0, 0.15);
   opacity: 1;
+  flex-grow: 1;
 }
 
 .section-divider-y {