fixes #3839

Uses width set to 800px for invite div instead of position fixed. It
looks the same for desktop and makes the button clickable on smaller
resolutions and mobile. Although it is not really responsive so the
text will be small on mobiles. Better that though than a non-clickable
button.

public/less/login.less

@@ -110,13 +110,9 @@
   text-align: center;
   border: 1px solid @grafanaTargetFuncBackground;
 	background-color: @grafanaPanelBackground;
-	position: fixed;
-  max-width: 800px;
-  left: 0;
-  right: 0;
+  width: 800px;
   margin-left: auto;
   margin-right: auto;
-  top: 20%;
 
   .tight-form {
     text-align: left;