copy_code_button: Attach tooltip to body to avoid overlap with parent.

Having tooltip `appendTo` to parent causes it to be trimmed by
the size of parent container if the parent doesn't have enough
size to include the tooltip. To fix this, we append tooltip
to `document.body`.

static/js/tippyjs.js

@@ -178,7 +178,7 @@ export function initialize() {
     });
 
     delegate("body", {
-        target: ".rendered_markdown time",
+        target: [".rendered_markdown time", ".rendered_markdown .copy_codeblock"],
         allowHTML: true,
         appendTo: () => document.body,
         onHidden(instance) {

static/templates/copy_code_button.hbs

@@ -1,3 +1,3 @@
-<button class="btn pull-left copy_button_base copy_codeblock tippy-zulip-tooltip" data-tippy-content="{{t 'Copy code' }}" aria-label="{{t 'Copy code' }}">
+<button class="btn pull-left copy_button_base copy_codeblock" data-tippy-content="{{t 'Copy code' }}" aria-label="{{t 'Copy code' }}">
     {{> copy_to_clipboard_svg }}
 </button>