Modularizes user roles.

In order to include specific user roles in applications or engines, we need to separate the user roles into modules.
Adds dutch translation.
Fixes issue with essence validation for uniqueness.

Validating the uniqueness of an essence always leaded to a failing validation because the way of finding a duplicate was wrong: We did not consider that we also find the essence that is currently validated.
Adds tests for validating the presence of essence ingredient.
Refactors Element.update_contents method.
Fixes issue where error message of essence was not set.

The error message got lost when using contents.find(id). Without doing that, everything works like a charm.