feat: display stats about total & active TUs on proposals

This patch brings in two new features:
- when viewing proposal listings, there is a new Statistics section,
  containing the total and active number of Trusted Users found in the
  database.
- when viewing a proposal directly, the number of active trusted users
  assigned when the proposal was added is now displayed in the details
  section.

Closes #323

Signed-off-by: Kevin Morris <kevr@0cost.org>

templates/partials/tu/proposal/details.html

@@ -21,6 +21,11 @@
         </strong>
     </div>
 
+    <div class="field">
+        {{ "Active" | tr }} {{ "Trusted Users" | tr }} {{ "assigned" | tr }}:
+        {{ voteinfo.ActiveTUs }}
+    </div>
+
     {% set submitter = voteinfo.Submitter.Username %}
     {% set submitter_uri = "/account/%s" | format(submitter) %}
     {% set submitter = '<a href="%s">%s</a>' | format(submitter_uri, submitter) %}