Add browse page helper

For business and benefits pages:
- correctly change margin-bottom on header and
- render action links

app/helpers/browse_helper.rb

@@ -0,0 +1,7 @@
+module BrowseHelper
+  SLUGS = %w[benefits business].freeze
+
+  def is_matching_slug?(slug)
+    SLUGS.include?(slug)
+  end
+end

app/views/browse/show.html.erb

@@ -20,7 +20,7 @@
   } %>
 <% end %>
 
-<%= render "shared/browse_header", { margin_bottom: [ "benefits", "business" ].include?(page.slug) ? 7 : 9 } do %>
+<%= render "shared/browse_header", { margin_bottom: is_matching_slug?(page.slug) ? 7 : 9 } do %>
   <h1 class="browse__heading govuk-heading-xl">
     <%= page.title %>
   </h1>
@@ -31,7 +31,7 @@
   } %>
 <% end %>
 
-<% if [ "benefits", "business" ].include?(page.slug) %>
+<% if is_matching_slug?(page.slug) %>
   <div class="govuk-width-container">
     <div class="govuk-grid-row">
       <% if page.slug == "benefits" %>