improvements

[SenCoder]

There is no change for function, just some improvements for code readability.

1. add IsValid & IsOnDuty method for UserGroup.
2. extract record struct for alerts handlers.

[jayryu]

Thanks for your attention and contribution, and I want to know have you ever tested your code?

[SenCoder]

yes, yet I didn't fix much code as it looks like, just reduce some repeated code, and because the use of element.IsValid() && element.IsOnDuty() actually reduce the indents, it looks like so many modifications