Get additional metadata from Mendeley to enhance GESIS data

[Joeran]

@MillahMau when fetching the readership statistics from mendeley's api, can you please have a look if we can also get additional metadata (especially the abstract) from mendeley? in that case we might use mendeleys' abstract if we do not have the abstract directly from gesis.

This ticket is only for looking if the can get the abstract. Later, we can still create another ticket for actually using the abstract.