cli: warn if applied and unapplied migrations are being squashed together

[rikinsk]

Is your proposal related to a problem?

In case some of the migrations being squashed are already applied and some are not, post squashing there is no way to apply the missing parts of the squashed migration to the db (other than running the SQL manually of course) and typically will lead to errors in the next migration apply.

Describe the solution you'd like

Allow squashing migrations only if all the involved migrations are either applied or unapplied. In case an inconsistency is observed, raise a warning to the user mentioning the problem this would create. Also suggest the solution, ie. apply the unapplied migrations on the database before squashing. Allow the user to ignore the warning in case they know what they are doing and have reasons to do so

Describe alternatives you've considered