Limited MOAT Solution Length to spec's 20 byte limit

[bitmold]

this is a small UI adjustment, the MOAT spec limits solution size to 20 bytes so I set a maxLength of 20 on it

From: https://gitweb.torproject.org/bridgedb.git/tree/README.rst#n391

Responding to a CAPTCHA challenge
"""""""""""""""""""""""""""""""""

To propose a solution to a CAPTCHA, the client MUST send a request for ``POST
/moat/check``, where the body of the request contains the following JSON::

    {
      "data": [{
        "id": "2",
        "type": "moat-solution",
        "version": "0.1.0",
        "transport": "TRANSPORT",
        "challenge": "CHALLENGE",
        "solution": "SOLUTION",
        "qrcode": "BOOLEAN",
      }]
    }


where:

* ``TRANSPORT`` is the agreed upon transport which will be distributed,
* ``CHALLENGE`` is a base64-encoded CAPTCHA challenge which MUST be
  later passed back to the server along with the proposed solution.
* ``SOLUTION`` is a valid unicode string, up to 20 bytes in length,
  containing the client's answer (i.e. what characters the CAPTCHA
  image displayed).  The solution is *not* case-sensitive.
* ``BOOLEAN`` is ``"true"`` if the client wants a qrcode containing the bridge
  lines to be generated and returned; ``"false"`` otherwise.