Add copy button to copy code block contents (#345)

* add buttons to copy code block contents

Adds a clickable "copy" link in the top-right corner of each code block.

If available, uses the navigator.clipboard API. Falls back to selecting
the text and calling document.execCommand('copy') to copy text.

* hides copy button unless mouse is hovering over code block

* change text of copy button when text is copied

* add translation keys for copy button text  `code_copy` and `code_copied`

* To disable use `Params.disableCodeCopy: true` in site config

assets/css/common/main.css

@@ -41,5 +41,25 @@
 }
 
 code {
-    direction: ltr
+    direction: ltr;
+}
+
+div.highlight {
+    position: relative;
+}
+
+.copy-code {
+    display: none;
+    position: absolute;
+    top: 4px;
+    right: 4px;
+    color: rgba(255, 255, 255, 0.8);
+    background: rgba(78, 78, 78, 0.8);
+    border-radius: var(--radius);
+    padding: 0 5px;
+    font-size: 14px;
+}
+
+div.highlight:hover .copy-code {
+    display: block;
 }