HTT header: attach header per-project instead of per-domain

[stsewd]

What's the problem this feature will solve?

Currently, we allow attaching http headers to custom domains only.

readthedocs.org/readthedocs/projects/models.py

Lines 1722 to 1726 in 0c1df75

	domain = models.ForeignKey(
	Domain,
	related_name='http_headers',
	on_delete=models.CASCADE,
	)

This has a couple of problems:

• If a project adds a new custom domain, we will need to create all current headers for that domain, and we will need to keep in sync all of them in case we change something.
• We can't have custom http headers for our default domains (readthedocs.io, readthedocs-hosted.com).

Describe the solution you'd like

Instead of attaching the header to a custom domain, we attach it to the project. I don't think there is a case a user will want to have different values per domain, if we find that there is a use case for that, we could keep the old field and support both options.

Alternative solutions

• Force people to always use a canonical custom domain if they want additional headers.
• Keep all custom domains from a project in sync manually or have an automatic way of doing it.
• Create a custom nginx configuration each time we want to add a header to all domains a project is served from.

[humitos]

This seems a duplicated of #8649. Feel free to move anything from this issue there if there are more context/considerations here that are not there.