Previously, when viewing the source of a locally-echoed (pending)
message, if one tried to copy-paste using the copy button, the
"Copied!" status was displayed but the message was not copied.
This was because as we use message ids with a . in then for locally
echoed messages, which ClipboardJS considered an error (invalid
selector).
To solve this
* Remove the data-clipboard-target attribute of copy button.
* Dynamically set target in ClipboardJS config
the based on message id using querySelector
with the message id written with CSS.escape.
Fixes#18053.
Since both the original button svg's have their own individual height
and width, this commit tweaks it to values height=20 and width=16 which
works well for both buttons.