Added support for "Informal participation in the contest"

[Heartfirey]

"Informal participation in the contest" in Chinses is "打星参赛". I recommend contest model add this function, which contest manager can set the “Star” label for some user. This will provide more detailed support for some contests. In contest, the "Star" user will have a yellow background on the list and will not participate in the total ranking when rolling out the list. At same time, if it's possible, please add identification label to exported file. Thank you very much!