add twitter card css

[pkra]

From https://github.com/mathblogging/mathblogging-base/issues/107

The following renders nicely

blockquote.twitter-tweet {
  display: inline-block;
  font-family: "Helvetica Neue", Roboto, "Segoe UI", Calibri, sans-serif;
  font-size: 12px;
  font-weight: bold;
  line-height: 16px;
  line-height: 1.33;
  border-color: #eee #ddd #bbb;
  border-radius: 5px;
  border-style: solid;
  border-width: 1px;
  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.15);
  margin: 1em;
  max-width: 468px;
  background-image: url("data:image/svg+xml;charset=utf-8,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20viewBox%3D%220%200%2072%2072%22%3E%3Cpath%20fill%3D%22none%22%20d%3D%22M0%200h72v72H0z%22%2F%3E%3Cpath%20class%3D%22icon%22%20fill%3D%22%231da1f2%22%20d%3D%22M68.812%2015.14c-2.348%201.04-4.87%201.744-7.52%202.06%202.704-1.62%204.78-4.186%205.757-7.243-2.53%201.5-5.33%202.592-8.314%203.176C56.35%2010.59%2052.948%209%2049.182%209c-7.23%200-13.092%205.86-13.092%2013.093%200%201.026.118%202.02.338%202.98C25.543%2024.527%2015.9%2019.318%209.44%2011.396c-1.125%201.936-1.77%204.184-1.77%206.58%200%204.543%202.312%208.552%205.824%2010.9-2.146-.07-4.165-.658-5.93-1.64-.002.056-.002.11-.002.163%200%206.345%204.513%2011.638%2010.504%2012.84-1.1.298-2.256.457-3.45.457-.845%200-1.666-.078-2.464-.23%201.667%205.2%206.5%208.985%2012.23%209.09-4.482%203.51-10.13%205.605-16.26%205.605-1.055%200-2.096-.06-3.122-.184%205.794%203.717%2012.676%205.882%2020.067%205.882%2024.083%200%2037.25-19.95%2037.25-37.25%200-.565-.013-1.133-.038-1.693%202.558-1.847%204.778-4.15%206.532-6.774z%22%2F%3E%3C%2Fsvg%3E");
  background-repeat: no-repeat;
  background-position: top 0.5rem right 0.5rem;
  background-size: 1.25rem 1.25rem;
  padding: 1rem 2rem 1rem 1rem;
}

blockquote.twitter-tweet p {
  font-size: 1.2em;
  font-weight: normal;
  line-height: 20px;
  line-height: 1.4;
}