Simplify using text color for links

Using `inherit` is IMHO more expressive since it means "use the color of
the parent element".

This is particularly useful for CONSUL installations using custom
styles. Consider the following code:

```
h2 {
  color: $white;

  a {
    color: $white;
  }
}
```

If we'd like to customize the way headings look, we'd have to override
two colors:

```
h2 {
  color: $red;

  a {
    color: $red;
  }
}
```

Consider the scenario where we use `inherit`:

```
h2 {
  color: $white;

  a {
    color: inherit;
  }
}
```

Now we only need to override one color to change the styles:

```
h2 {
  color: $red;
}
```

app/assets/stylesheets/admin/menu.scss

@@ -2,6 +2,7 @@
   background: $sidebar;
   background: linear-gradient(to bottom, #245b80 0%, #488fb5 100%);
   border-right: 1px solid $border;
+  color: $white;
 
   ul {
     list-style-type: none;
@@ -22,7 +23,6 @@
     }
 
     [class^="icon-"] {
-      color: #fff;
       display: inline-block;
       font-size: rem-calc(20);
       line-height: $line-height;
@@ -136,7 +136,7 @@
   }
 
   li a {
-    color: #fff;
+    color: inherit;
     display: block;
     line-height: $line-height * 2;
     padding-left: $line-height / 4;
@@ -144,7 +144,7 @@
 
     &:hover {
       background: $sidebar-hover;
-      color: #fff;
+      color: inherit;
       text-decoration: none;
     }
   }