Add ability to omit basePath in Reference fields

[fzaninotto]

Problem

When using a Field component outside of a Datagrid or a Form, users have to inject props manually. They can understand that they must inject record and resource, but they often forget to inject basePath. It is not a problem, except for Reference fields, where the basePath is used to build the links to the related records. Using these Fields without a basePath leads to a cryptic error:

TypeError: Cannot read property 'replace' of undefined

The basePath property isn't well documented, and to be honest, it isn't very useful. In the most common case, the basePath equals /${resource}.

Solution

If the basePath isn't set, use the resource name instead. That way, users can omit the basePath when using ReferenceField, ReferenceArrayField, and ReferenceManyField manually.

    <ReferenceField
        resource="comments"
        record={data[id]}
        source="post_id"
        reference="posts"
-       basePath="/comments"
    >
        <TextField source="title" />
    </ReferenceField>