Add bibliographic reference to entities

[PGijsbers]

Try to produce a list of bibliographic references which use (or cite) the entity, for example all papers that use the AutoML Benchmark study /s/269. And/or allow people to add this information themselves.

[PGijsbers]

Allow citation information metadata to be stored on OpenML entities, so that they may easily be exported to e.g., bibtex.
This makes it easier to cite e.g., studies or datasets.
Consider using the Citation File Format: https://citation-file-format.github.io/

For this we need:

a change to the database to store this data
possibly an update to the server API to allow storage of this data
a button on the front-end that allows exports
export functions in packages