Make large fractions actually work

Summary:
Also, rename it to '\dfrac', because it's more like a display fraction

Auditors: spicyj

static/style.css

@@ -104,3 +104,8 @@ input {
     height: 0.05em;
     min-height: 1px;
 }
+
+.mfracnum > span {
+    display: inline-block;
+    vertical-align: bottom;
+}