Support different citation formats

[mgrani]

The citation format should depend on the resource type:

1. Bibliographic resources should use citation styles. See the wordpress plugin or contact Nils Witt on how to integrate that.
2. Other resources should be simply added using a link. If the resource has an image preview, add the image also.

Users might want to change between those two.

[chseifert]

Citations should only be automatically included for scholarly articles