Improve DB fixtures

[marksteward]

The DB fixtures need a bit of attention, all independent but worth a look at together:

• The code to add permissions, venues, tags, bank accounts, etc should be moved out of the startup path
• Perhaps there should be a first-run script that does do this when the DB is empty
• Running dev data should be idempotent or at least warn if it's already run, and should refuse to run on production
• The village generation code can currently generate the same village name twice, which throws a unique error
• The dev data script has no error handling
• Product group generation should be moved out to yaml or similar, and should include examples of merch, sponsorship, hires, etc. It'd be nice to be able to export at the end of a year and compare the output with what's in git.
• Use schema or jsonschema to validate fixtures (including the volunteer init_data)

[russss]

Running dev data should be idempotent or at least warn if it's already run

I think I have very occasionally needed to run this multiple times to add a bit more data. I think if we can find a sensible way to trigger "first run" actions then that should be sufficient.

Product group generation should be moved out to yaml or similar, and should include examples of merch, sponsorship, hires, etc. It'd be nice to be able to export at the end of a year and compare the output with what's in git.

We already export the product group structure as JSON in the post-event export (it's in the private export, although I'm not entirely sure if it needs to be) - so we should just stick a copy of that in the repo (lightly sanitised if necessary) and create from that for dev. It would be nice to also create the ProductViews in a production-like state at the same time.