FAP/Proposals and assignments displays only one of the instruments for proposals with multiple instruments

[gnyiri]

Steps to reproduce:

• Create a proposal with more than one instrument associated
• Assign the proposal to a FAP
• Look for the proposal in "FAP/Proposals and assignments": only one of the instruments will be shown in the table
• Filter the table for each instrument assigned -> the proposal will be shown correctly

Also a technical question:

• why do we store instrument_has_proposal_id in the fap_proposals table?