Improve factories for external contributors

Recent discussion we had about where to put factories code:

@brodock :

how should factories be organized? we have both spec/factories/*.rb and spec/factories.rb(edited)

@rspeicher :

@gabriel: I know! It drives me crazy. Originally we only had spec/factories.rb, but I think the model generator creates a separate one for each model, so we ended up with a few of those.

spec/factories/project.rb is probably big enough that it warrants its own file, so I was thinking we should just go all-in and have one file for each model class

It's been on my todo list for a while 😦

The important thing is that we be consistent, so you're not stuck going "is this factory I want to edit in factories.rb or its own file?"

Some of the spec/factories/model_name.rb got there by the use of related generators (rspec:models for example).

@sytses proposes that if we don't want new factories to go to spec/factories.rb we should add a notice there to explain where new code should go.

If we want to keep single files the exception, but still allow the creation of them, I believe a README.md in spec/factories could give some additional instructions when to use one or the other.