Enable toc sidebar scrolling (mmistakes#2874)

* Enable toc sidebar scrolling

* Refactor style rules

* Move style rules from 'navigation' to 'sidebar'

* Remove custom scrollbar styles

Author: j3soon

_sass/minimal-mistakes/_navigation.scss

@@ -200,8 +200,8 @@
       margin-left: 0;
     }
   }
-  
-  img{
+
+  img {
     -webkit-transition: none;
     transition: none;
   }

_sass/minimal-mistakes/_sidebar.scss

@@ -89,6 +89,13 @@
       position: sticky;
       top: 2em;
       float: right;
+
+      .toc {
+        .toc__menu {
+          overflow-y: auto;
+          max-height: calc(100vh - 7em);
+        }
+      }
     }
   }