API key management on admin UI

[tusmester]

Developers can use API keys for authentication (in case of 3rd party services this is the only option). Add a new section to the API and security settings page where administrators can manage API keys.

Add a new tab here:

Tutorial: https://docs.sensenet.com/tutorials/authentication/how-to-authenticate-apikey

The backend OData API offers the following methods: https://github.com/SenseNet/sensenet/blob/master/src/Services.Core/Operations/ApiKeyOperations.cs