Fix issue with PR build hostname parsing

[agjohnson]

Given a hostname slug--bug--1.org.readthedocs.build, the current
parsing code will give a project slug of slug and a version slug of
bug--1. This was a valid slug creation pattern for a short while,
though we have changed the behavior of the slugging to not produce slugs
with consecutive dashes.

rsplit here searches for the right most instance of double slugs

Fixes #8699

[stsewd]

Would be good to add a test for this, around here

readthedocs.org/readthedocs/proxito/tests/test_full.py

Lines 154 to 173 in 2cab501

	@override_settings(
	RTD_EXTERNAL_VERSION_DOMAIN='dev.readthedocs.build',
	)
	def test_external_version_serving(self):
	fixture.get(
	Version,
	verbose_name='10',
	slug='10',
	type=EXTERNAL,
	active=True,
	project=self.project,
	)
	url = '/awesome.html'
	host = 'project--10.dev.readthedocs.build'
	resp = self.client.get(url, HTTP_HOST=host)
	self.assertEqual(
	resp['x-accel-redirect'], '/proxito/media/external/html/project/10/awesome.html',
	)
	# Invalid tests

Also, do you have the problematic project that caused this? So we can QA after deploy.

[agjohnson]

Yeah, I wanted to write a quick test, but ran into issues running the test suite.

This was a commercial project that hit the issue by the way, I'll ping you on the issue.

[agjohnson]

Also, I might not return to this until later in my afternoon. Feel free to take the wheel here on the test case if you'd like to get this merged sooner!

[stsewd]

I'll add a test for this tomorrow