Fixed issue boards scrolling with a lot of lists & issues

When a board has a lot of lists & issues scrolling stops the user from moving the issue to the lsat list (or any list not on screen). This changes that by making the scrollable element the board-list element. This will need re-thinking when sorting in lists is possible.

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: