Indicate expected outcome for undispatched tasks in the patch page

We recently reduced the frequency of waterfall builders for the ! and * builders. One consequence of that is that developers are now more likely to find that the "base" tasks remain undispatched.

It would be helpful if undispatched tasks came with an xfail / xpass indication, showing what the result of the most recent execution of the task on the waterfall was.

This isn't perfect - intervening commits between the most recent run on the waterfall and the base commit for the patch may have introduced novel failures. These indications could be visual (pale green, pale pink) or textual (put xfail or xpass) in the text after undispatched, or both.