Show login page if the URL has campaign parameter for anon users
For non-logged-in users show login page instead of nosuchspecialpage
if the URL has the campaign parameter.
After logging in the user should be taken to Special:CX
again with all the query parameters.
This is important for campaigns like email campaigns, to prevent
a situation where the user who clicked is not be logged in
and doesn't see anything relevant.
Showing nosuchspecialpage is a bad idea in that case.