Made DeviceRoles hierarchical, had to also change the filtersets for Device, ConfigContext and VirtualMachine to use the TreeNodeMultipleChoiceFilter.
Note: The model was changed to use NestedGroupModel, a side-effect of this is it also adds comments field, but I thought that was better then doing a one-off just for DeviceRole and having to define the fields, validators, etc.. - keeps everything DRY / consistent.
* 18981 Make Device Roles Hierarchical
* 18981 forms, serializer
* 18981 fix tests
* 18981 fix tests
* 18981 fix tests
* 18981 fix tests
* 18981 fix tests
* 18981 fix migration merge
* 18981 fix tests
* 18981 fix filtersets
* 18981 fix tests
* 18981 comments
* 18981 review changes
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