Add tag/category to the commitfest app

Hi,

During the last commit fest, some new contributors mentioned me that they were
having trouble to find patch to review.

And indeed, the commit fest app doesn't provide any kind of information about
the patch difficulty (which is different from the patch size, so you can't even
try to estimate it that way), whether there's a need for acceptance on the
feature/approach more than a code review, if it's a POC..., and that's not
welcoming.

I was thinking that we could add a tag system, probably with some limited
predefined tags, to address that problem.

Do you think it's worth adding, or do you have a better idea to help newer
contributors?