Use CSS to display icons in the admin menu

This way we simplify the HTML and generating similar menus will be
easier. We also improve the experience for screen reader users, who
might have been hearing the icons as text because we weren't using the
`aria-hidden` attribute.

We're still keeping the "icon-" classes for compatibility with CONSUL
installations which might have changed this code.

app/assets/stylesheets/admin/menu.scss

@@ -8,6 +8,20 @@
     margin-bottom: 0;
     margin-left: 0;
     padding: 0;
+  }
+
+  > ul > li a {
+    font-weight: bold;
+
+    @mixin icon($name, $style) {
+      @include has-fa-icon($name, $style);
+
+      &::before {
+        font-size: rem-calc(20);
+        margin-left: rem-calc(8);
+        margin-right: rem-calc(10);
+      }
+    }
 
     [class^="icon-"] {
       color: #fff;
@@ -18,10 +32,81 @@
       vertical-align: middle;
       width: $line-height * 1.5;
     }
-  }
 
-  > ul > li a {
-    font-weight: bold;
+    &.booths-link {
+      @include icon(archive, solid);
+    }
+
+    &.budgets-link,
+    &.investments-link {
+      @include icon(chart-pie, solid);
+    }
+
+    &.comments-link {
+      @include icon(comments, regular);
+    }
+
+    &.dashboard-link {
+      @include icon(check, solid);
+    }
+
+    &.debates-link {
+      @include icon(comment-alt, solid);
+    }
+
+    &.invitations-link {
+      @include icon(envelope, regular);
+    }
+
+    &.legislation-link {
+      @include icon(file-alt, solid);
+    }
+
+    &.messages-link {
+      @include icon(file-archive, regular);
+    }
+
+    &.moderated-content-link {
+      @include icon(eye, regular);
+    }
+
+    &.polls-link {
+      @include icon(check-circle, regular);
+    }
+
+    &.print-proposals-link,
+    &.print-investments-link {
+      @include icon(print, solid);
+    }
+
+    &.profiles-link {
+      @include icon(users, solid);
+    }
+
+    &.proposals-link,
+    &.proposal-notifications-link {
+      @include icon(lightbulb, solid);
+    }
+
+    &.settings-link {
+      @include icon(cog, solid);
+    }
+
+    &.signature-sheets-link {
+      @include icon(file-alt, regular);
+    }
+
+    &.site-customization-link {
+      @include icon(pencil-alt, solid);
+    }
+
+    &.stats-link {
+      @include icon(chart-line, solid);
+    }
+
+    &.users-link {
+      @include icon(user, solid);
+    }
   }
 
   li {