add /tu/{proposal_id} (get, post) routes

This commit ports the `/tu/?id={proposal_id}` PHP routes to
FastAPI into two individual GET and POST routes.

With this port of the single proposal view and POST logic,
several things have changed.

- The only parameter used is now `decision`, which
  must contain `Yes`, `No`, or `Abstain` as a string.
  When an invalid value is given, a BAD_REQUEST response
  is returned in plaintext: Invalid 'decision' value.
- The `doVote` parameter has been removed.
- The details section has been rearranged into a set
  of divs with specific classes that can be used for
  testing. CSS has been added to persist the layout with
  the element changes.
- Several errors that can be discovered in the POST path
  now trigger their own non-200 HTTPStatus codes.

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

templates/partials/tu/proposal/form.html

@@ -0,0 +1,14 @@
+<form class="action-form" action="/tu/{{ proposal }}" method="POST">
+    <!-- Translate each button's text but leave the value alone. -->
+    <fieldset>
+        <button type="submit" class="button" name="decision" value="Yes">
+            {{ "Yes" | tr }}
+        </button>
+        <button type="submit" class="button" name="decision" value="No">
+            {{ "No" | tr }}
+        </button>
+        <button type="submit" class="button" name="decision" value="Abstain">
+            {{ "Abstain" | tr }}
+        </button>
+    </fieldset>
+</form>