Add maybe-query and model-or-models helpers

[jeffdaley]

Introduces two LinkTo-focused helpers to make the DropdownList::LinkTo component more flexible.

Example usage:

<LinkTo
    {{! fall back to an empty hash if @query is undefined }}
    @query={{maybe-query @query}}
    
    {{! fall back to an empty array if no models are provided }}
    @models={{model-or-models @model @models}}