Add "copy to clipboard" button for code blocks (#2812)

* Add copy-to-clipboard button and JS

* Ignore line numbers if present

* Rewrite heading permalink code to use vanilla JS

* README: Add credits to zenorocha/clipboard.js (MIT License)

@iBug really wants a place here in the Credits section :P

* Add .no-copy for hiding the button, update docs

* Add td.rouge-code to selectors

* Fix navigator.clipboard branch

* Add screenreader text for copy button

* Restore focus to the button after copying

* Add site-wide enable switch

_sass/minimal-mistakes/_utilities.scss

@@ -591,3 +591,60 @@ a.reversefootnote {
     position: static;
   }
 }
+
+/*
+   Copy <pre> block to clipboard
+   ========================================================================== */
+
+// a <textarea> to hold text for document.execCommand("copy")
+.clipboard-helper {
+  // Prevent zooming on iOS
+  font-size: 12pt !important;
+  border: 0 !important;
+  padding: 0 !important;
+  margin: 0 !important;
+  outline: none !important;
+  position: absolute;
+}
+
+pre {
+  .clipboard-copy-button {
+    display: block;
+    position: absolute;
+    top: 0.6em;
+    right: 0.5em;
+    z-index: 1;
+    background: none;
+    border: none;
+    outline: none;
+    border-radius: 0.1em;
+    padding: 0.2em 0.5em;
+    color: white;
+    opacity: 0.4;
+    transition: color 0.25s linear -0.25s, opacity 0.25s linear;
+
+    &:hover {
+      color: #ffffca;
+    }
+
+    &::before {
+      content: '';
+      position: absolute;
+      top: 0;
+      right: 0;
+      bottom: 0;
+      left: 0;
+      z-index: 2;
+    }
+
+    @at-root {
+      .no-copy & {
+        display: none;
+      }
+    }
+  }
+
+  &:hover .clipboard-copy-button {
+    opacity: 1;
+  }
+}