Add copy button to markdown code blocks (#17638)

* Add copy button to markdown code blocks

Done mostly in JS because I think it's better not to try getting buttons
past the markup sanitizer.

* add svg module tests

* fix sanitizer regexp

* remove outdated comment

* vertically center button in issue comments as well

* add comment to css

* fix undefined on view file line copy

* combine animation less files

* Update modules/markup/markdown/markdown.go

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

* add test for different sizes

* add cloneNode and add tests for it

* use deep clone

* remove useless optional chaining

* remove the svg node cache

* unify clipboard copy string and i18n

* remove unused var

* remove unused localization

* minor css tweaks to the button

* comment tweak

* remove useless attribute

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.ini

@@ -85,6 +85,12 @@ remove = Remove
 remove_all = Remove All
 edit = Edit
 
+copy = Copy
+copy_url = Copy URL
+copy_branch = Copy branch name
+copy_success = Copied!
+copy_error = Copy failed
+
 write = Write
 preview = Preview
 loading = Loading…
@@ -927,13 +933,6 @@ fork_from_self = You cannot fork a repository you own.
 fork_guest_user = Sign in to fork this repository.
 watch_guest_user = Sign in to watch this repository.
 star_guest_user = Sign in to star this repository.
-copy_link = Copy
-copy_link_success = Link has been copied
-copy_link_error = Use ⌘C or Ctrl-C to copy
-copy_branch = Copy
-copy_branch_success = Branch name has been copied
-copy_branch_error = Use ⌘C or Ctrl-C to copy
-copied = Copied OK
 unwatch = Unwatch
 watch = Watch
 unstar = Unstar