Add the ability to load/save a plan

When a plan is computed, the user can select/unselect items, add pages to the exclude list, etc...

If the user come back after, all her settings and the plan are lost.

I think we should:

• save the plan in an XObject, serialized with JSON
• being able to reload the plan from that XObject, and resume the migration if it has been suspended

Since this plan could be saved, we could even imagine doing the migration in a staging server first, and then en production with the exact same plan.

It can also be good if the migration has failed (because of XWIKI-9046) to continue where it has failed without computing a new plan.