title identifiers have no fixed order

The array of title identifiers in a TitleInstance has no fixed order.

As there is the possibility to have more than one identifier of the same namespace and the decision for 'display only one identifier' was made, this behaviour results in not displaying the same value for this identifier namespace reliably.

To reproduce this, you need one local eresource with at least 2 eissn (or other namespace) identifiers and reload the eresources list and / or the eresource detail view page in the UI.

fix sorting / ordering in the backend so that title identifiers are always returned in the same order