Fix Citation modal responsiveness and clipboard copy

[silverwind]

The modal was broken in two ways:

• On small screens, the input box was partially hanging outside the modal. Fixed with flexbox and increased modal width.
• The clipboard copy was not working because the modal had both data-clipboard-text and data-clipboard-target, while we only support one of those. Made a small tweak in clipboard as well so that it will still fall back to target if text is empty.

[wxiaoguang]

I do not think this could be backported, some code is 1.22-only.

[silverwind]

It should theoretically apply to 1.21, not many changes around these parts. But yeah I'm not thrilled about another manual backport. Let's still see whether GiteaBot can do it.

[wxiaoguang]

It can't, because I am sure the "copy" code was changed by #29379 , it is only in 1.22

[silverwind]

Ah I see, #29379 had broken this copy button. The style fix is barely worth backporting imho.