fix login page vertical scroll and centering

src/skins/vector/css/templates/Login.css

@@ -17,6 +17,18 @@ limitations under the License.
 .mx_Login {
     width: 100%;
     height: 100%;
+
+    display: -webkit-box;
+    display: -moz-box;
+    display: -ms-flexbox;
+    display: -webkit-flex;
+    display: flex;
+    -webkit-align-items: center;
+    align-items: center;
+    -webkit-justify-content: center;
+    justify-content: center;
+
+    overflow: auto;
 }
 
 .mx_Login h2 {
@@ -28,8 +40,8 @@ limitations under the License.
 
 .mx_Login_box {
     width: 300px;
+    min-height: 450px;
     margin: auto;
-    padding-top: 100px;
 }
 
 .mx_Login_logo {