Add security to mutations

[hsteinmueller]

Summary of change

Added spring-security with ADMIN role. There is no use for a USER role by now since queries should be allowed for everyone. If it is need it can be added easily.

Mutations are only allowed for ADMIN while queries are allowed for everyone.

Related issue

Closes #69

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[knjk04]

Hi @hsteinmueller, thanks for working on this. Unfortunately, this violates our contributing guidelines, as we request that you first request to be assigned to avoid duplicated work (someone is already assigned to this issue).

If this issue becomes available (if the person assigned to it doesn't respond within our time frame and the next person who enquiried on the issue also doesn't want to work on this or doesn't respond, we can offer this to you).