Clean up 'Flow talk page manager' related code
- Always use the hardcoded English version so the username is the same
across all wikis.
- Use accounts even if we didn't create them. If a malicious user takes
the name, they will be locked out due to the UserGetReservedNames
- Previous detection only checked if the user was a bot, which can easily be spoofed.
- If CentralAuth is installed, attach new users to the global account.
- Remove special casing for PHPUnit tests :)