Move migrate_data stuff from custom migration to management command

[asadiqbal08]

Description:

As per some findings over the PR: Django Upgrade to 3.2 and referring to this comment. It seems, we should move the logic of migrate_data from custom migration file to Django management command.
Drawback is that, we have to update the migration files for dependencies manually whenever there is a model update in third party applications such as Wagtail.

@pdpinch FYI

[asadiqbal08]

@pdpinch a question, should I have to work on this or keep it on hold ?

[pdpinch]

We should do it now, while you and Arslan remember the details. I will set the relative priority on the team board.