Redmine Colored Enumeration

When colored enumerations are disabled, the multi-selection feature should be enabled again.

Currently, the multi-selection field is always disabled:

Fix tests failing due to the changing behavior and write new tests making sure

• the checkbox for multi-selection is available when color support is disabled and vice versa
• issues with a key/value custom field having no color support but multi-selection will show multiple values when the user did assign them