Allow the option to perform a dry-run when using the CCMA

Currently, when using the CCMA, the migrations follow the trial and error approach, where it's all or nothing.

It would be useful to be able to perform a dry-run so that users can predetermine what are the errors they might run into.

Example:
When migrating spaces where the attachments can't be found, you can have multiple missing attachments and until you resolve all of them, the migration will keep failing.