Proposal: change `Project`'s `project_funder` to `funder`

[PGijsbers]

I propose to change the attribute name of project_funder to funder for the Project concept.
I do not see the benefit for the "project_" prefix, and it is not used for other attributes (e.g., "participant" and "coordinator" are used instead of "project_participant" and "project_coordinator"). Unless there is a good reason to keep it, I propose to drop the prefix to preserve 'symmetry' with the other properties.