Center modals and fix sizing

Previously the modals were not centered on the screen and the size
of the modal was set to 50% of the screen, leading to a lot of white
space within modals with less content. This sizes the modal based on
the amount of content within and centers the modal. Also adds the grey
modal overlay to the 'confirm network settings' modal to be consistent
with the other pages.

Resolves openbmc/phosphor-webui#35

Tested: Viewed all modals currently in GUI (Firmware page, network page,
        and overview page) and modals were sized correctly and appeared
        with the modal overlay in the background.

Change-Id: Ie35d179810d36b8bcbbf39dc1302e773387fa788
Signed-off-by: beccabroek <beccabroek@gmail.com>
diff --git a/app/common/styles/elements/modals.scss b/app/common/styles/elements/modals.scss
index 396b46e..1a8b71f 100644
--- a/app/common/styles/elements/modals.scss
+++ b/app/common/styles/elements/modals.scss
@@ -1,34 +1,36 @@
 .modal-overlay {
-  width:100%;
-  height:100%;
-  z-index:-1;
-  background-color:#000;
-  position:fixed;
-  top:0;
-  left:0;
-  margin:0;
-  padding:0;
+  width: 100%;
+  height: 100%;
+  z-index: -1;
+  background-color: #000;
+  position: fixed;
+  top: 0;
+  left: 0;
+  margin: 0;
+  padding: 0;
   opacity: 0;
 }
 
 .modal-overlay.active{
-  z-index:100;
+  z-index: 100;
   opacity: .5;
   display: block;
   @include fastTransition-all;
 }
 
 .modal {
-  width:50%;
+  width: auto;
+  height: auto;
+  left: 50%;
+  top: 50%;
+  transform: translate(-50%,-50%);
   max-width: 850px;
-  margin: .5em auto;
+  min-width: 450px;
   padding: .8em 1.5em;
   border: thin #000 solid;
-  background-color:#fff;
-  z-index:101;
-  position:fixed;
-  top:-100%;
-  left:25%;
+  background-color :#fff;
+  z-index: 101;
+  position: fixed;
   overflow: hidden;
   @include fastTransition-all;
   @include mediaQuery(medium) {
@@ -37,7 +39,6 @@
 }
 
 .modal.active{
-  top: 25%;
   @include fastTransition-all;
   display: block;
 }
@@ -47,11 +48,11 @@
 }
 
 .screen-reader-offscreen {
-  position:absolute;
-  left:-999px;
-  width:1px;
-  height:1px;
-  top:auto;
+  position: absolute;
+  left: -999px;
+  width: 1px;
+  height: 1px;
+  top: auto;
 }
 .modal__content {
   border-bottom: 1px solid $lightbg__grey;