Toggling between 'All' and 'Custom' in 'Configure Fields' drop-down blanks fields set as readonly by behaviours

Steps to reproduce

• Set up a text custom field.
• Edit an issue so that it has a non-empty value for the field.
• Set up a behaviour that makes the field readonly.
• Open the edit screen on the issue from before. The custom field's value is shown in the corresponding form field, which is greyed out.
• Click 'Configure Fields' to open the drop-down.
• Toggle the 'Show Fields' option from 'All' to 'Custom' or vice versa.

Expected behaviour: The custom field value is preserved in the form.

Actual behaviour: The value is set to empty and, since the form field is readonly, it cannot be edited back to its old value. The issue edit has to be cancelled if you wish to avoid persisting the blank value.

(This might bear some relation to SRJIRA-233 but the symptoms appear different.)

Proposed Solution

It is expected from a custom field to loose it's value after it gets "un-configured" from the screen.
In that case, we have to do something similar, like remove all configured behaviours from a field when this field gets "un-configured" from the screen.