Bulk actions in the backlog

Grooming the backlog no longer means editing tickets one by one.

• Select many. Tick tickets individually, select a whole sprint or the backlog from its header, or select everything at once.
• Act on all of them. A bar appears with actions to move tickets to a sprint, assign them, set their status, or flag them.
• Check before it happens. Every bulk action shows the affected tickets and asks you to confirm first.
• You decide who can. By default only admins can use bulk actions. Admins can allow other members to use them for the whole organization or per project.

Also fixed: abandoned tickets no longer disappear when moved from a sprint back to the backlog.