Do not build projects from banned users

[humitos]

If any owner of the project is banned, we skip building this project.

[ericholscher]

Seems reasonable. I do worry about false positives here, where a valid user is banned, perhaps because a spammer added them to a spam project. Having no feedback on why your project isn't building is bad UX, but so is telling the spammer that we know they are a spammer. :/

[humitos]

I do worry about false positives here, where a valid user is banned, perhaps because a spammer added them to a spam project

When we "ban the owner" of a project with multiple users, an exception is risen because of this reason.

https://github.com/rtfd/readthedocs.org/blob/b07f4550345b90e07fd2504b984c3361a05a97c6/readthedocs/projects/admin.py#L165-L167

The other case is true, but it's unlikely: "a valid user add a spammer user into a valid project".

Having no feedback on why your project isn't building is bad UX, but so is telling the spammer that we know they are a spammer. :/

I do see two different ways of giving feedback here (not for this PR though):

1. Add a permanent message that appears while is_active == True
2. Create the Build object with a pre-filled error without triggering the Celery task

(I remember that we used to have the message hardcoded in the template for project.skip == True but we removed it because we said that was a bad pattern and we wanted to use our notification system)

[ericholscher]

Cool. Let's ship this and we can worry about the side effects later, I think. Hopefully we shouldn't be banning users accidentally :)