Scroll collapsed file into view (#23702) (#23929)

Backport #23702 by @jpraet

Fixes #23701, #23515.

Alternate approach to #23604 using CSS scroll-margin-top, which is also
taken into account for direct links to files in a diff:

* On the PR diff, this currently shows the previous file first:
https://try.gitea.io/jpraet/test/pulls/13/files#diff-b94d08b409f9d05fb65b6cccaf7b3e4ecc7cc333
* On the commit diff, the first line of the linked file is currently
under the sticky header:
https://try.gitea.io/jpraet/test/commit/1a19e6b14e31d295b7372f3346580c3e85690ff5#diff-b94d08b409f9d05fb65b6cccaf7b3e4ecc7cc333

Co-authored-by: Jimmy Praet <jimmy.praet@ksz-bcss.fgov.be>

web_src/css/repository.css

@@ -1862,6 +1862,13 @@
 .diff-file-box {
   border: 1px solid transparent;
   border-radius: var(--border-radius);
+  scroll-margin-top: 47px; /* match .repository .diff-detail-box */
+}
+
+@media (max-width: 991px) {
+  .diff-file-box {
+    scroll-margin-top: 77px; /* match .repository .diff-detail-box */
+  }
 }
 
 /* TODO: this can potentially be made "global" by removing the class prefix */