Add modal look to authentication flows

This changes the auth screens to use the modal-like style of the redesign.

This does not attempt to style the actual body content of each screen.  Instead,
it covers the header area with logo, footer links, and overall modal container
only.

res/css/views/auth/_AuthHeader.scss

@@ -14,12 +14,16 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+.mx_AuthHeader {
+    width: 206px;
+    padding: 25px 50px;
+    box-sizing: border-box;
+}
+
 .mx_AuthHeader_logo {
-    text-align: center;
-    height: 150px;
-    margin-bottom: 45px;
+    margin-top: 15px;
 }
 
 .mx_AuthHeader_logo img {
-    max-height: 100%
+    width: 100%;
 }