Adding non-numeric participant role values breaks event inf page and participant listing

If you create a participant role which has a non numeric value, it breaks the event info pages and several event full checks.
I can reproduce this on the 4.6 and master demo site.

This is because the check on roles_is faulty assumes that the participant role values are integers

IMO there are two approaches to fix this:

• force numeric values for the participant_role option_values when added through UI and API.
• don't use the custom values for participant roles
• don't assume the participant role ids are numeric

The last approach is IMO "better" (but still not optimal), as enforcing numeric values through the API would add a lot of overhead to the customvalue.create logic, and rewriting the whole participant_role logic would be quite big.

I will create a PR with the last solution.