Support multiple surveys per participant

[adamrp]

More checkboxes to be added (maybe; add an issue for each checkbox):

• Logging samples has to be per-survey (it is currently per-participant) (Logging samples when there are multiple surveys #280)
• Navbar interface should have an extra layer (hover over human source -> human -> survey -> samples) (Add navbar interface for multiple surveys per participant #279)
• Add survey_name to ag_login_surveys table (mandatory, varchar, no foreign key, unique constraint on (ag_login_id, participant_name, survey_name)) to be displayed in the menu above (Add survey_name to ag_login_surveys #278)