Fixed issue boards issue sorting when dragging issue into list

## What does this MR do?

Currently it just appends the new issue to the end of list & then sorts by priority which can cause some strange effects. For example if you drag the issue to the top of the list & then vue re-renders, the issue actually goes to the bottom.

This changes that by adding the issue at the specific index.

## Screenshots (if relevant)

![boards-sorting](/uploads/9aa4a0cb5e0be79e7d24ab2b6abb0bbb/boards-sorting.gif)

See merge request !7734