oro_entity_config:
    email:
        field:
            items:
                available_in_template:
                    options:
                        default_value:      false
                        is_bool:            true
                    form:
                        type:               oro_entity_extend_choice
                        options:
                            choices:        ['No', 'Yes']
                            empty_value:    false
                            block:          other
                            label:          'Available in email templates'

oro_entity_config:
    email:
        field:
            items:
                available_in_template:
                    options:
                        default_value:      false
                        is_bool:            true
                        allowed_type:       [string,integer,smallint,bigint,boolean,decimal,date,text,float,money,percent,optionSet,file,image,ref-one,manyToOne]
                    form:
                        type:               oro_entity_extend_choice
                        options:
                            choices:        ['No', 'Yes']
                            empty_value:    false
                            block:          other
                            label:          'Available in email templates'

Disable available in templates flag for Many2Many, One2Many relation
CRM-1639: Disable available in templates flag for Many2Many, One2Many relation
