Unify poster preview with poster PDF styles

Since commit c5103d3025, the styles from application.scss no longer apply
to app/views/dashboard/poster/index.pdf.erb.

To recover the text-center alignment, we add the rule in dashboard.scss.

Note that we also remove text-center from the h2, since it wasn't applied
and there's no need to recover it because it doesn't exist in the preview
either.

app/assets/stylesheets/dashboard.scss

@@ -545,6 +545,10 @@
     }
   }
 
+  .proposal-code {
+    text-align: center;
+  }
+
   .poster-content {
     margin: 0 auto;
     max-width: 90%;

app/views/dashboard/poster/index.html.erb

@@ -13,7 +13,7 @@
           <%= sanitize(t("dashboard.poster.index.intro_text", org: Setting["org_name"])) %>
         </p>
 
-        <p class="text-center">
+        <p class="proposal-code">
           <strong><%= t("dashboard.poster.index.proposal_code", code: proposal.code) %></strong>
         </p>
         <div class="proposal-image">

app/views/dashboard/poster/index.pdf.erb

@@ -18,7 +18,7 @@
     <p class="intro">
       <%= sanitize(t("dashboard.poster.index.intro_text", org: Setting["org_name"])) %>
     </p>
-    <p class="text-center proposal-code">
+    <p class="proposal-code">
       <strong><%= t("dashboard.poster.index.proposal_code", code: proposal.code) %></strong>
     </p>
     <div class="proposal-image">
@@ -29,7 +29,7 @@
       <% end %>
     </div>
     <div class="poster-content">
-      <h2 class="text-center"><strong><%= t("dashboard.poster.index.support") %></strong></h2>
+      <h2><strong><%= t("dashboard.poster.index.support") %></strong></h2>
       <%= wicked_pdf_image_tag "quote_before_blue.png", alt: "" %>
       <h3><strong><%= proposal.title %></strong></h3>
       <%= wicked_pdf_image_tag "quote_after_blue.png", alt: "" %>