Add the total number of issues in the JSON response in issue board lists

Add the total number of issues in the JSON response in issue board lists

The issue board lists should always show the total number of issues in the list, not the current amount fetched by endless scroll.

Closes #21327

See merge request !5904
Conflicts:
	app/assets/stylesheets/pages/boards.scss
	app/views/projects/boards/components/_board.html.haml