Project slugs with multiple dashes can't load PR builds

[agjohnson]

For a period of time, projects could end up with a slightly invalid slug, with double dashes. This could happen if the project name included a utf8 character that would not slugify or even a dash/underscore. New projects seem to slug properly now, but for any project that was created in the period where these slugs were possible, PR builds will not work due to pattern matching bugs.

The logic here simply breaks up the first --, which for a project slug like slug--bug--1.org.readthedocs.build, would not parse correctly. Bug is here:

readthedocs.org/readthedocs/proxito/middleware.py

Line 82 in bc3e147

project_slug, version_slug = host_parts[0].split('--', 1)

[agjohnson]

I put up a PR, but not entirely sure it resolves the bug.