GSoC: allow proposals to be associated to multiple projects

[vvolkl]

Example of where his creates problems: https://hepsoftwarefoundation.org/gsoc/2026/proposal_Geant4_fastsim_Data.html

[sm-28601]

Example of where his creates problems: https://hepsoftwarefoundation.org/gsoc/2026/proposal_Geant4_fastsim_Data.html

Hi Valentin, Pls assign me this issue . I would love to contribute .

[vvolkl]

Example of where his creates problems: https://hepsoftwarefoundation.org/gsoc/2026/proposal_Geant4_fastsim_Data.html

Hi Valentin, Pls assign me this issue . I would love to contribute .

Thanks, please, go ahead!