`CustomField::OrderStatements` handles multi-value custom fields for formats `string`, `text`, `bool`, `link` and `date`. UI doesn't allow setting multi value for those formats, also there doesn't seem to be any restriction in controller or model. Also `CustomField#multi_value_possible?` lists only `version`, `user` and `list` as possible for multi-value. It also doesn't make sense to be able to order by text (long text) custom fields.

Should `CustomField` validate multi value field and should we remove handling of multi value from order statements? Begin of the insertion

Acceptance criteria:

* It is still possible to create custom fields of types `string`, `test`, `bool`, `link` and `date`

* It is still possible to create custom fields of types `version`, `user` and `list` both allowing and not allowing multiple valuesEnd of the insertion