Split ElementType management component

Preliminary work for #802 (closed): this splits the ElementType management form into the same structure as the one of AllowedMetaData so that only one type is being managed at a time. This makes adding the deletion button much easier without having to rely on strange data attributes.

assigned to @erwanrouchet

changed the description

requested review from @babadie

mentioned in merge request !1085 (merged)

mentioned in commit 21a412dc

merged