feat(aio): copy code snackbar and design updates

- Add snackbar and pointer cursor for copy code button inside aio-code components
- Flex cenetered content in features page
- Removed duplicate global css class
- Add styles to links inside of sub-sections
- Remove focus outline on top nav bar links

aio/src/styles/1-layouts/_layout-global.scss

@@ -27,9 +27,6 @@ l-relative {
   flex-wrap: wrap;
 }
 
-.flex-center {
-    justify-content: center;
-}
 .flex-center {
     display: flex;
     justify-content: center;

aio/src/styles/1-layouts/_top-menu.scss

@@ -3,9 +3,15 @@
   flex: 1 1 auto;
 }
 
-.nav-link {
+aio-top-menu a.nav-link {
   margin: 0 16px;
   cursor: pointer;
+  
+  &:focus {
+    background-color: $accentblue;
+    outline: none;
+    padding: 21px 16px;
+  }
 }
 
 .nav-link.home img {

aio/src/styles/2-modules/_code.scss

@@ -91,6 +91,7 @@ aio-code.headed-code {
   color: $lightgray;
   background-color: transparent;
   border: none;
+  cursor: pointer;
   &:hover {
     color: $mediumgray;
   }

aio/src/styles/2-modules/_subsection.scss

@@ -1,11 +1,16 @@
 .l-sub-section {
     color: $darkgray;
-    background-color: $lightgray;
-    border-left: 8px solid $mediumgray;
+    background-color: rgba($blue, 0.05);
+    border-left: 8px solid $blue;
     padding: 16px;
     margin-bottom: 8px;
 
     h3 {
         margin: 8px 0 0;
     }
+
+    a:hover {
+        color: $blue;
+        text-decoration: underline;
+    }
 }