Documents: Add invalid document server template

Invalid document template is now served or included from
a specific template file. Documents with invalid
API image URLs now return None instead of the template code
specific '#'.  The new template is called invalid_document.html.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/documents/templates/documents/invalid_document.html

@@ -0,0 +1,8 @@
+<div class="text-center" style="border: height: 100%;">
+    <div class="fa-3x">
+        <span class="fa-layers fa-fw">
+            <i class="far fa-file text-primary"></i>
+            <i class="fa-inverse fas fa-times text-danger" data-fa-transform="shrink-6"></i>
+        </span>
+    </div>
+</div>