Add ability for LinkChecker to check authed links

Because the preview environment is hidden behind HTTP basic auth, link checking will fail for any links to other pages on the site in this environment. This change allows us to configure domains that require authentication such that the LinkChecker will be able to automatically apply the auth credentials when required. This will be configured at deploy time so that environment-specific details are kept out of the codebase.

https://www.agileplannerapp.com/boards/173808/cards/4822