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>

web/html/css/aurweb.css

@@ -204,3 +204,11 @@ label.confirmation,
 	overflow: hidden;
 	transition: height 1s;
 }
+
+.proposal.details {
+    margin: .33em 0 1em;
+}
+
+button[type="submit"] {
+    padding: 0 0.6em;
+}