Refactor to use a heading class for height when using header-illustrations

This fixes a glitch highlighted in #3259, where the css class stopped applying
when the view was moved to a different controller.

We're moving to having each view responsible for their layout, and commonly having
override classes explicitly in the relevant views. So we can follow that pattern here.

app/assets/stylesheets/common.scss

@@ -1035,13 +1035,8 @@ tr.turn:hover {
 
 /* Overrides for pages that use new layout conventions */
 
-.users-new,
-.users-create,
-.users-terms,
-.users-confirm {
-  .content-heading .content-inner {
-    height: 200px;
-  }
+.header-illustration-heading {
+  height: 200px;
 }
 
 .header-illustration {

app/views/confirmations/confirm.html.erb

@@ -1,3 +1,4 @@
+<% content_for :heading_class, "header-illustration-heading" %>
 <% content_for :heading do %>
   <h1><%= t ".heading" %></h1>
   <div class='header-illustration confirm-main'></div>

app/views/users/blocked.html.erb

@@ -1,3 +1,4 @@
+<% content_for :heading_class, "header-illustration-heading" %>
 <% content_for :heading do %>
   <h1><%= t "users.new.title" %></h1>
   <div class='header-illustration new-user-main'></div>

app/views/users/new.html.erb

@@ -2,6 +2,7 @@
   <%= javascript_include_tag "user" %>
 <% end %>
 
+<% content_for :heading_class, "header-illustration-heading" %>
 <% content_for :heading do %>
   <h1><%= t ".title" %></h1>
   <div class='header-illustration new-user-main'></div>

app/views/users/terms.html.erb

@@ -2,6 +2,7 @@
   <%= javascript_include_tag "user" %>
 <% end %>
 
+<% content_for :heading_class, "header-illustration-heading" %>
 <% content_for :heading do %>
   <h1><%= t ".heading" %></h1>
   <div class='header-illustration new-user-terms'></div>