Add the ability to interrupt a migration

[raphj]

It can be useful to kill / terminate a migration process when investigating a bug. There's currently no easy way to do it in the confluence migrator application.

It could be implemented with a "stop" button on migration pages.

Important: the temporary files created by the confluence xml package in the work directory should be cleaned up.

[snazare]

cancel button on Migrations livetable Actions
cancel option in Migration page, import step