Feature/multi objective

[langdal]

This PR implements first steps towards #68 The focus of this PR is to implement the core functionality and leave the UI design to another issue.

Doing

• [x] Support storing multiple scores for each data line
• [x] Allow entry of multiple scores
• [x] Display plots in both modes
• [x] Preserve normal single-objective functionality when multi-objective mode is disabled
• [x] Handle migration of existing experiment JSON files
• [x] Keep multi objective scores when flipping switch to single objective mode
• [x] Respect mode when sending data to backend
• [x] Disable switch at deploy time
• [x] Revert api container in dev container to use the main branch
• [x] Save experiment after CSV upload

Not doing

• Decide how multi objective feature fits into the UI
• Add better Pareto visualization, e.g., through Bokeh #142