switch tweet embed centering to recommended method:

https://developer.twitter.com/en/docs/twitter-for-websites/embedded-tweets/overview

assets/sass/components/_content.scss

@@ -70,13 +70,6 @@ div#content {
       }
     }
 
-    &.tweet {
-      .twitter-tweet-rendered {
-        margin: 1em auto;
-        display: block;
-      }
-    }
-
     &.youtube {
       position: relative;
       padding-bottom: 56.25%;