Fix Citation modal responsiveness and clipboard copy (#29799)

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.

web_src/css/repo.css

@@ -2035,13 +2035,8 @@
 }
 
 #cite-repo-modal #citation-panel {
-  width: 500px;
-}
-
-@media (max-width: 767.98px) {
-  #cite-repo-modal #citation-panel {
-    width: 100%;
-  }
+  display: flex;
+  width: 100%;
 }
 
 #cite-repo-modal #citation-panel input {
@@ -2061,6 +2056,7 @@
   padding: 5px 10px;
   font-size: 1.2em;
   line-height: 1.4;
+  flex: 1;
 }
 
 #cite-repo-modal #citation-panel #citation-copy-apa,