Make the admin menu fill the screen vertically

On high-resolution screens where neither the menu nor the main content
were filling the screen, there was a blank space at the bottom which
looked weird.

app/assets/stylesheets/admin.scss

@@ -22,6 +22,7 @@ $sidebar-hover:  #25597c;
 $sidebar-active: #f4fcd0;
 
 .admin {
+  @include admin-layout;
 
   h2 {
     font-weight: 100;

app/assets/stylesheets/dashboard.scss

@@ -16,6 +16,10 @@
 // 01. Dashboard global
 // --------------------
 
+.proposal-dashboard {
+  @include admin-layout;
+}
+
 .proposal-title {
   display: inline-block;
 

app/assets/stylesheets/mixins.scss

@@ -3,6 +3,7 @@
 // 01. Logo
 // 02. Orbit bullets
 // 03. Direct uploads
+// 04. Admin layout
 // ------------------
 
 // 01. Logo
@@ -126,6 +127,18 @@
   }
 }
 
+// 04. Admin layout
+// ----------------
+
+@mixin admin-layout {
+  display: flex;
+  flex-direction: column;
+
+  > .menu-and-content {
+    flex: 1;
+  }
+}
+
 @mixin has-fa-icon($icon, $style) {
   @extend .fa-#{$icon};