Codechange: use std::string without const as return

Otherwise some compilers, e.g. MSVC, do not pick up that these are temporaries
and as such it will pass the temporaries to `const std::string &` instead of
the wanted `std::string &&`

Codechange: use GSCompanyMode::IsValid, IsDeity, and the precondition helpers

Direct 1:1 replacements in the code, and comments now refer to either
GSCompanyMode::IsValid or GSCompanyMode::IsDeity instead of several variations
on "company mode active" or "no company mode active".