Update Button disabled color to grey50 according to guidelines (#1465)

Author: ethanshar

src/components/button/ButtonConstants.ts

@@ -23,4 +23,4 @@ export const MIN_WIDTH = {
 };
 
 export const DEFAULT_SIZE = ButtonSize.large;
-export const DISABLED_COLOR = Colors.dark60;
+export const DISABLED_COLOR = Colors.grey50;

src/components/button/__tests__/__snapshots__/index.spec.js.snap

@@ -217,7 +217,7 @@ exports[`Button backgroundColor should return theme disabled color if button is
   style={
     Object {
       "alignItems": "center",
-      "backgroundColor": "#C2C7CB",
+      "backgroundColor": "#D2D6D8",
       "borderRadius": 999,
       "flexDirection": "row",
       "justifyContent": "center",
@@ -1913,7 +1913,7 @@ exports[`Button icon should return icon style according to different variations
   style={
     Object {
       "alignItems": "center",
-      "backgroundColor": "#C2C7CB",
+      "backgroundColor": "#D2D6D8",
       "borderRadius": 999,
       "flexDirection": "row",
       "justifyContent": "center",
@@ -2770,7 +2770,7 @@ exports[`Button labelColor should return disabled text color according to theme
             "lineHeight": 24,
           },
           Object {
-            "color": "#C2C7CB",
+            "color": "#D2D6D8",
           },
           Object {},
           Object {},
@@ -3302,7 +3302,7 @@ exports[`Button outline should return disabled color for outline if button is di
             "lineHeight": 24,
           },
           Object {
-            "color": "#C2C7CB",
+            "color": "#D2D6D8",
           },
           Object {},
           Object {},