FIX Correct bugs with Basic Auth option

[sig-peggy]

Description

Fixes multiple issues preventing use of the Basic Auth configuration

Manual testing steps

1. Set up the Basic Auth environment variables
2. Access the dev/check page as both:
   1. A logged in Admin user - expected behaviour is that the user will not need to enter any additional credentials to view the page
   2. Without being logged in - expected behaviour is that Basic Auth authentication will be available for the username and password set up in step 1 only

Issues

• Issues when using HTTP BASIC Auth on EnvironmentChecker.php #73
• Using basic auth credentials to access /dev/check still triggers the site login #92
• Turning on Basic Auth credentials prevents authorised users from accessing /dev/check #116

Pull request checklist

• The target branch is correct
  • See picking the right version
• All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
  • Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• The commit messages follow our commit message guidelines
• The PR follows our contribution guidelines
• Code changes follow our coding conventions
• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[GuySartorelli]

Closing in favour of #119