Fix button spacing, Add image placeholders

sass/components/nav.sass

@@ -1,4 +1,4 @@
-$nav-height: 3rem !default
+$nav-height: 3.5rem !default
 
 // Components
 
@@ -20,7 +20,7 @@ $nav-height: 3rem !default
     flex-grow: 1
     flex-shrink: 0
   img
-    max-height: 1.5rem
+    max-height: 1.75rem
   .button + .button
     margin-left: 0.75rem
   .tag
@@ -48,11 +48,11 @@ a.nav-item
     padding-right: 1rem
     padding-top: calc(0.5rem - 1px)
     &:hover
-      border-bottom-color: $link
+      border-bottom-color: $primary
       border-top-color: transparent
     &.is-active
-      border-bottom: 3px solid $link
-      color: $link
+      border-bottom: 3px solid $primary
+      color: $primary
       padding-bottom: calc(0.5rem - 3px)
 
 // Containers