DATAREST-721 - Show input fields for associations in CustomPostForm …

[marcelkauf]

… to be able to create resources with mandatory associations.

Input field for associations are now shown. These are only simple text fields, the user is responsible for inserting the correct URI. It is only possible to enter one URI, additional links have to be added via separate requests, e.g. PATCH. Jackson has to be configured properly for this to work (spring.jackson.deserialization.accept-empty-string-as-null-object,
spring.jackson.deserialization.accept-single-value-as-array). This is just a simple fix but definitely an improvement since it wasn't possible to create resources with mandatory associations before.

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