Track reviewers in panels for code review systems

Currently, in panels for code review systems (Gerrit, GitHub Pull Requests), we are tracking mainly submitters of patches. We should track (maybe in different panels) reviewers, since they are also a very interesting part of the story. For this, I think we have all the information in the raw indexes, so very likely we don't need changes in `Perceval`. But for sure we will need changes in `GrimoireELK` (maybe for creating a new index with info about reviewers) and in `panels` (for the new panels, or modification of the current ones).