Add user id to visualizations

Fix user authentication decoding

Only temporary. Will be added again later on.

Add owner validation

Rename server.js to app.js.
Add 404 and error handling middleware.

Add rows and columns to plot

Post routes create needed relationships when creating new entries.
The associated entires are deleted on deleting an entry (e.g. deleting a project
leads to deleting all visualizations and plots belonging to this project.

Add relationship from visualization to project.
Add plot-type attribute.