Make user suggestions and project suggestions work in the same way

[nicolasfranck]

Right now adding contributors to a publication/dataset, and adding projects to a publication/dataset behave a bit differently:

• You can add a user multiple times to a publication. Is that a problem?
• You cannot add a project multiple times to a publication. If you search for a project that is already in the list, nothing warns you that it is, and if you click on "add project", nothing seems to have happened because the project was silently removed and added back again (see here);

Suggestion:

• in the search modal: mark users as already in the list (cf. pinterest does this with "Psst you already saved this")