Does not remove the issue from the selected tab when it it de-selected, it instead gets purged when changing tab

Fixed scroll position being lost

Selected data is added into a different array

Changed what is hidden when data is loading

Adds issues to list on frontend - need to push to backend

Fixed modal not closing on cancel clicked

Submit button disables/enables
Submit button selects ids - but does not push it yet

Possibly need to change masonry library but so far it is the only one I found that works well

This removes the backlog list & instead creates a modal window that will
list all issues that are not part of a list for easy adding onto the
board

Closes #26205

PlantUML support for Markdown

Closes #4048

See merge request !8588

Backport changes from EE squash

See merge request !8912