Move evaluation related stuff to viewmodel

Already mentioned [here](https://github.com/kir-dev/pek-next/pull/93#pullrequestreview-191479182), it would be nicer using a viewmodel for all these stuff. If you check [this](https://github.com/kir-dev/pek-next/blob/master/app/views/evaluations/table.html.erb) there are too much logic in this template imho, so some kind of viewmodel should be designed and used every evaluation related place.

Start by reviewing app/views/evaluations/table.html.erb and the discussion linked from pull request #93. Trace the other evaluation-related places mentioned by the issue and identify the logic that should move into a shared viewmodel. Done means evaluation views use the designed viewmodel rather than keeping this logic in templates.

Written by the indexing model from the issue text.