This commit renames the "Huddle" Django model class to
"DirectMessageGroup", while maintaining the same table --
"zerver_huddle".
Fixes part of #28640.
We now have helpers for the two places where
we create databases.
There was already one helper in place, and
I gave it a more concrete name, to match
its actual database name in postgres.
So `generate-fixtures` only ever did 9 lines of code (really
3 lines of actual code) in its normal mode of operation.
But it was cluttered with lots of stuff that really only
happened when you called it with the `-force` option, which
was only invoked by `rebuild-test-database`.
Now we inline most of the code into `rebuild-test-database`.
And now `generate-fixtures` is simple (and doesn't support
a `-force` flag.
The new tools now have more concise, more parallel names:
- rebuild-dev-database
- rebuild-test-database
The actual implementations are still pretty different:
rebuild-dev-database:
mostly delegates to 5 management scripts
rebuild-test-database:
is a very thin wrapper for generate-fixtures
We'll try to clean that up a bit soon.