Maximum length on keys and display names

https://redmine.teklia.com/issues/12585, ARKINDEX-BACKEND-304

In the backend, the key and display_name fields are limited to 250 characters, while the help_text has no limit. Because previous configurations only had a title, and no division between display_name and help_text, the title contained the text for both, which could get particularly long. The conversion script moves title to display_name only, and because the backend trusts the validator to give it a good configuration, it does not check for the maximum length and explodes.

The spec should state that key and display_name are limited to 250 characters. The validator should check for this, and the migration script should use help_text instead of display_name when a title gets too long.