Merge branch 'issue-boards-scrollable-element' into 'master'

Fixed issue boards scrolling with a lot of lists & issues

## What does this MR do?

When an issue board has a lot of lists & each list has a lot of issues scrolling can be tricky. This changes that by making the element that scrolls the `board-list` div. This is ok because sorting inside each list isn't possible, it will need re-thinking when sorting is possible but this fixes the bug for now.

See merge request !7748

app/assets/javascripts/boards/components/board_list.js.es6

@@ -80,6 +80,7 @@
     },
     mounted () {
       const options = gl.issueBoards.getBoardSortableDefaultOptions({
+        scroll: document.querySelectorAll('.boards-list')[0],
         group: 'issues',
         sort: false,
         disabled: this.disabled,

changelogs/unreleased/issue-boards-scrollable-element.yml

+---
+title: Fixed issue boards scrolling with a lot of lists & issues
+merge_request: 
+author: