Add visual separators: inverted banner sections on receipts, dividers in navbar

ReceiptBuilder#invert (GS B) + #section_separator give the test page (and
future receipts) a full-width white-on-black banner between sections --
the closest a direct-thermal printer gets to a second color. Navbar gets
a vr divider between Home | Brother/Shipping labels/Receipts | Labels to
group the three logical clusters.

views/_navbar.erb

@@ -9,6 +9,7 @@
         <li class="nav-item">
           <a class="nav-link <%= 'active' if request.path == '/' %>" href="/">Home</a>
         </li>
+        <li class="nav-item"><div class="vr mx-2 my-2"></div></li>
         <li class="nav-item">
           <a class="nav-link <%= 'active' if request.path.start_with?('/brother') %>" href="/brother">Brother</a>
         </li>
@@ -18,6 +19,7 @@
         <li class="nav-item">
           <a class="nav-link <%= 'active' if request.path == '/epson' %>" href="/epson">Receipts</a>
         </li>
+        <li class="nav-item"><div class="vr mx-2 my-2"></div></li>
         <li class="nav-item">
           <a class="nav-link <%= 'active' if request.path.start_with?('/labels') %>" href="/labels">Labels</a>
         </li>