Fix HTTP to HTTPS redirection not working for default domains

If a default domain is in use, the group domain will be passed into the IsHTTPSOnly() call, but this will always return false because the HTTPS-only flag can only be determined by looking at the HTTP request host rather than the URL.

For example, for the project https://gitlab.com/tanukitalks/tanukitalks.gitlab.io, this is what happened previously:

1. For a project in the default domain (e.g. gitlab.io), GitLab Pages loads config.json and sees the group is tanukitalks with the project name tanuitalks.gitlab.io. It stores the HTTPS-only flag inside the project config.
2. Note that for projects in the default domain, the domainConfig is empty. This makes sense because there is no domain configuration specified since the default domain is being used, and we need to redirect on a project-by-project basis.
3. User requests https://tanukitalks.gitlab.io.
4. GitLab Pages looks up tanukitalks.gitlab.io, and it returns the domain tanukitalks with an empty domainConfig and project tanukitalks.gitlab.io.
5. Since there is no domainConfig, IsHTTPSOnly attempts to resolve the project from the URL.
6. However, since the URL is using the default domain, the path is /, which doesn't resolve to any project.

In the new behavior, we check the hostname of the request to see if it matches any project before trying to parse the URL.

Closes #162 (closed)