unify short description styling (#4548)

apply same style class to the About description on both the landing page
and the about/more page

app/javascript/styles/about.scss

@@ -484,7 +484,7 @@
     padding: 0;
   }
 
-  .learn-more-cta {
+  .about-short {
     background: darken($ui-base-color, 4%);
     padding: 50px 0;
   }

app/views/about/more.html.haml

@@ -35,6 +35,7 @@
               %i.fa.fa-external-link{ style: 'padding-left: 5px;' }
 
       .container.hero
+        .about-short
           .heading
             %h3= t('about.description_headline', domain: site_hostname)
             %p= @instance_presenter.site_description.html_safe.presence || t('about.generic_description', domain: site_hostname)

app/views/about/show.html.haml

@@ -58,7 +58,7 @@
                 = @instance_presenter.closed_registrations_message.html_safe
             = link_to t('about.find_another_instance'), 'https://joinmastodon.org/', class: 'button button-alternative button--block'
 
-  .learn-more-cta
+  .about-short
     .container
       %h3= t('about.description_headline', domain: site_hostname)
       %p= @instance_presenter.site_description.html_safe.presence || t('about.generic_description', domain: site_hostname)