Better display of failing checks

[unho]

Right now Pootle editor only exposes which checks are failing, but some of the checks (for example accelerators check from TTK https://github.com/translate/translate/blob/master/translate/filters/checks.py#L1085-L1136) are able to return multiple error messages.

This can be improved if when clicking on a failing check it shows the specific error messages (instead of going to its description) with a rollout that lists the error messages below the check. Another option is to display those messages only when hovering (using a tooltip). Future improvement can be to display on the editor textarea where those errors happened.

This of course will require saving additional data on the QualityCheck model.