Make backup of user DB when carrying out a migration

[DillonJ]

During the Mopo meeting this week, I believe we had concensus that we should make backup of a user's SpineOpt DB when carrying out a migration.

I think it's ok to do it by default and without asking but it was suggested to give prompt the user whether they want this or not. The problem is a user may not fully appreciate the reasons for backing up at the time and migt make a bad decision.

Anyway - one way or another, we should provide for this functionlity, prompt or not.

Issue https://github.com/spine-tools/SpineOpt.jl/issues/837 relates