Add Cancel/Abort action to backup operations

[DavidePrincipi]

The “Run backup” and "Restore an application" operations of Backup page currently lacks a visible Cancel or Abort button.

• An immediate backup run can be started with one click, there is no way to stop it if started accidentally.
• If many restore are started in parallel, node load may raise too much.

Proposed improvement

• Add a Cancel (or Abort) action during manual backup and restore execution.
• Clearly communicate the backup state and whether it can be safely interrupted.

Acceptance conditions

For Run backup:

• An interrupted backup run must not raise any "Failed backup" alert.
• Any change to Restic repository should be simply discarded or not available for restore.
• Backup prune operation is not executed.

For Restore an application:

• Termination of the restore will remove the restored instance completely (remove-module starts in response to SIGTERM)

See also

• The Clone/Move task can be canceled. A similar pattern is acceptable.
• https://mattermost.nethesis.it/nethesis/pl/p8uu8moi47rbxcmg58ersjcywc

[stephdl]

see proposal videos in #7811 (comment)

[stephdl]

this will be verified in #7810