Fix padding in hero container of landing page (#4373)

* Fix padding in hero container, landing page

Erase hero container padding to fit registration form to full width. By this setting, heading padding disappears, so I adjust heading padding.

* Specify attribute strictly for heading padding

To overwrite padding-bottom in superior settings.

* Make padding shorthand more concise

app/javascript/styles/about.scss

@@ -708,6 +708,11 @@
 
       .hero {
         margin-top: 30px;
+        padding: 0;
+
+        .heading {
+          padding: 0 20px 20px;
+        }
       }
 
       .floats {