Currently it is not possible to create monolingual string datavalues longer than 400 chars.

This is currently not configurable. It seems to be set by ValidatorBuilders.php#298 which just uses the default length which is hardcoded to 400 (ValidatorBuilders.php#181)

Acceptance Criteria
Configuration option to define arbitrary length monolingual string values for snaks
Given that the user tries to make a string value shorter than (or equal to) the defined max length the edit is successful
Given that the user tries to make a longer value than the defined max length the edit fails with a validation error.