Remove the custom HSTS Domain settings

[ericholscher]

We currently have custom HSTS-specific settings in the Domain object. We should migrate these to the normal Header configuration that we have for all our other headers:

readthedocs.org/readthedocs/projects/models.py

Lines 1798 to 1812 in c4996c2

	# Strict-Transport-Security header options
	# These are not exposed to users because it's easy to misconfigure things
	# and hard to back out changes cleanly
	hsts_max_age = models.PositiveIntegerField(
	default=0,
	help_text=_('Set a custom max-age (eg. 31536000) for the HSTS header')
	)
	hsts_include_subdomains = models.BooleanField(
	default=False,
	help_text=_('If hsts_max_age > 0, set the includeSubDomains flag with the HSTS header')
	)
	hsts_preload = models.BooleanField(
	default=False,
	help_text=_('If hsts_max_age > 0, set the preload flag with the HSTS header')
	)

readthedocs.org/readthedocs/projects/models.py

Line 1854 in c4996c2

class HTTPHeader(TimeStampedModel, models.Model):

[akordata]

I will work on this!

[isdanni]

@ericholscher can you show an example of what a normal header looks like? From #6953 it looks like HSTS related header config is added directly without changing anything.