Primary commit for this MR:
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>