Introduce checks for merge identity

https://gitlab.kit.edu/kit/reg-app/regapp/-/blob/main/regapp-idty/src/main/java/edu/kit/scc/webreg/service/identity/IdentityUpdater.java#L109

Check primaray and secondary identity for equality. They have to be different.

Same on the merge-identity.xhtml page. Do not allow merge the same identities.