Add ability to 'squash' migrations

[zacps]

After the development process it would be useful to squash all migrations into one file.

[mpyw]

Pending since 2017... Any updates?

[rubenv]

@mpyw well obviously it hasn't been done yet.

Happy to accept any pull request that does this, but the first thing we need to figure out how it will actually work.

And as to why there's no progress after 7 years: nobody is getting paid for doing this, nor are you paying for it, so nobody is under any obligation to do it.

The best way to move things forward, as with any open-source project, is to do it yourself or sponsor someone who can do it for you.