[SPARK-6973] remove skipped stage ID from completed set on the allJobsPage

Though totalStages = allStages - skippedStages is understandable. But consider the problem [SPARK-6973], I think totalStages = allStages is more reasonable. Like "2/1 (2 failed) (1 skipped)", this item also shows the skipped num, it also will be understandable.

Author: Xu Tingjun <xutingjun@huawei.com>
Author: Xutingjun <xutingjun@huawei.com>
Author: meiyoula <1039320815@qq.com>

Closes #5550 from XuTingjun/allJobsPage and squashes the following commits:

a742541 [Xu Tingjun] delete the loop
40ce94b [Xutingjun] remove stage id from completed set if it retries again
6459238 [meiyoula] delete space
9e23c71 [Xu Tingjun] recover numSkippedStages
b987ea7 [Xutingjun] delete skkiped stages from completed set
47525c6 [Xu Tingjun] modify total stages/tasks on the allJobsPage