Improve issue list filter (#24425)

Partial regression of #24393, not only regression, but broken for long
time, 24393 didn't really improve it but used wrong `overflow: scroll`.

Actually, that "ui secondary filter menu labels" shouldn't be set as
scrollable (I missed that at that time), the problem is: if a "ui menu"
has "dropdown" items, then it should not be scrollable. Otherwise the
dropdown menu can't be shown correctly.

And there are more problems:

* The "issue-filters" shouldn't be used anywhere else (copying&pasting
problem again ....)
* There is also an "issue-actions" container, it should also be fixed.
* There are similar problems on the milestone page.
* The old comment in code: "grid column" doesn't work well.

The major changes of this PR are: use "flex: 1" instead of "ui grid
column".

After this PR, not 100% perfect but much better than before.

web_src/js/features/repo-issue-list.js

@@ -23,7 +23,7 @@ function initRepoIssueListCheckboxes() {
     toggleElem($('#issue-filters'), !anyChecked);
     toggleElem($('#issue-actions'), anyChecked);
     // there are two panels but only one select-all checkbox, so move the checkbox to the visible panel
-    $('#issue-filters, #issue-actions').filter(':visible').find('.column:first').prepend($issueSelectAll);
+    $('#issue-filters, #issue-actions').filter(':visible').find('.issue-list-toolbar-left').prepend($issueSelectAll);
   };
 
   $issueCheckboxes.on('change', syncIssueSelectionState);