Add separate component for post-auth security flows

Instead of twisting `AuthBody`, this adds a new component for the different
styling of post-auth security flows. This also makes them fixed width and
adjusts padding to match designs.

res/css/structures/auth/_CompleteSecurity.scss

@@ -22,7 +22,7 @@ limitations under the License.
 .mx_CompleteSecurity_headerIcon {
     width: 24px;
     height: 24px;
-    margin: 0 4px;
+    margin-right: 4px;
     position: relative;
 }