Collapse sidebar navigation list on smaller screens

- CSS only solution using "check-box hack"
- Add new UI text label `menu_label` in _data/ui-text.yml
- Close #607

_sass/_base.scss

@@ -290,6 +290,8 @@ nav {
   }
 
   a {
+    display: block;
+    padding-bottom: 0.25em;
     text-decoration: none;
   }
 

_sass/_navigation.scss

@@ -314,19 +314,51 @@
    ========================================================================== */
 
 .nav__list {
-  font-size: 1.25rem;
+  margin: 2em 0 1.5em 0;
 
-  ul {
-    margin-bottom: 1em;
+  input[type="checkbox"],
+  label {
+    display: none;
   }
 
-  a {
-    display: block;
-    padding: 0.125em 0;
-    color: inherit;
+  @include breakpoint(max-width $large) {
 
-    &:hover {
-      text-decoration: underline;
+    label {
+      position: relative;
+      display: inline-block;
+      padding: 0.5em 1em;
+      font-size: $type-size-6;
+      border: 1px solid $light-gray;
+      border-radius: $border-radius;
+      z-index: 20;
+      -webkit-transition: 0.2s ease-out;
+      transition: 0.2s ease-out;
+      cursor: pointer;
+
+      &:hover {
+        color: #fff;
+        border-color: $gray;
+        background-color: mix(white, #000, 20%);
+      }
+    }
+
+    /* selected*/
+    input:checked + label {
+      color: white;
+      background-color: mix(white, #000, 20%);
+    }
+
+    ul {
+      margin-bottom: 1em;
+    }
+
+    a {
+      display: block;
+      padding: 0.125em 0;
+
+      &:hover {
+        text-decoration: underline;
+      }
     }
   }
 
@@ -345,6 +377,37 @@
   }
 }
 
+.nav__list .nav__items {
+  margin: 0;
+  font-size: 1.25rem;
+
+  @include breakpoint(max-width $large) {
+    position: relative;
+    max-height: 0;
+    opacity: 0;
+    overflow: hidden;
+    z-index: 10;
+    -webkit-transition: 0.3s ease-in-out;
+    transition: 0.3s ease-in-out;
+    -webkit-transform: translate(0, 10%);
+        -ms-transform: translate(0, 10%);
+            transform: translate(0, 10%);
+  }
+}
+
+@include breakpoint(max-width $large) {
+  .nav__list input:checked ~ .nav__items {
+    -webkit-transition: 0.5s ease-in-out;
+    transition: 0.5s ease-in-out;
+    max-height: 100vh;
+    opacity: 1;
+    margin-top: 1em;
+    -webkit-transform: translate(0, 0);
+        -ms-transform: translate(0, 0);
+            transform: translate(0, 0);
+  }
+}
+
 .nav__title {
   margin: 0;
   padding: 0.5rem 1rem;

_sass/_sidebar.scss

@@ -33,10 +33,6 @@
     font-family: $sans-serif-narrow;
   }
 
-  h3, h4 {
-    font-size: $type-size-5;
-  }
-
   p, li {
     font-family: $sans-serif;
     font-size: $type-size-6;