Fix double scroll bar in admin content

We were using `overflow: scroll` as a workaround with a problem we had
with the equalizer. But now we never need an extra vertical scroll bar,
and we only need an extra horizontal scroll bar on small screens.

Since the dashboard was using the class `admin-content` as well, we need
to apply to the dashboard the same changes we've done in the admin
section. I've extracted them into a mixin.

app/assets/stylesheets/admin.scss

@@ -292,21 +292,10 @@ $sidebar-active: #f4fcd0;
   }
 
   .side-menu-and-admin-content {
-    display: flex;
-  }
-
-  .side-menu {
-    flex: 25%;
-    min-width: 250px;
-
-    .admin-sidebar {
-      height: 100%;
-    }
+    @include side-menu-and-content;
   }
 
   .admin-content {
-    flex: 75%;
-
     .proposal-form {
       padding-top: 0;
     }
@@ -325,11 +314,6 @@ $sidebar-active: #f4fcd0;
   display: none;
 }
 
-.admin-content {
-  overflow: scroll;
-  padding: $line-height !important;
-}
-
 @include breakpoint(medium) {
 
   tr {

app/assets/stylesheets/dashboard.scss

@@ -16,6 +16,10 @@
 // 01. Dashboard global
 // --------------------
 
+.dashboard-menu-and-content {
+  @include side-menu-and-content;
+}
+
 .proposal-title {
   display: inline-block;
 
@@ -288,10 +292,6 @@
   background: #fbfbfb;
   border-right: 1px solid $border;
 
-  @include breakpoint(medium) {
-    min-height: $line-height * 45;
-  }
-
   [class^="icon-"] {
     color: $text;
     display: inline-block;

app/assets/stylesheets/mixins.scss

@@ -1,8 +1,9 @@
 // Table of Contents
 //
 // 01. Logo
-// 02. Orbit bullets
-// 03. Direct uploads
+// 02. Side menu and content
+// 03. Orbit bullets
+// 04. Direct uploads
 // ------------------
 
 // 01. Logo
@@ -21,7 +22,29 @@
   }
 }
 
-// 02. Orbit bullet
+// 02. Side menu and content
+// -------------------------
+
+@mixin side-menu-and-content {
+  display: flex;
+
+  > :first-child {
+    flex: 25%;
+    min-width: 250px;
+
+    > :first-child {
+      height: 100%;
+    }
+  }
+
+  > :last-child {
+    flex: 75%;
+    overflow-x: auto;
+    padding: $line-height !important;
+  }
+}
+
+// 03. Orbit bullet
 // ----------------
 
 @mixin orbit-bullets {
@@ -49,7 +72,7 @@
   }
 }
 
-// 03. Direct uploads
+// 04. Direct uploads
 // ------------------
 
 @mixin direct-uploads {