Resolve "Add log level config option"

Closes #6, #7, #10, #14, and #15

See merge request !5

Fix mongoose promise library deprecation

Fix file log level
Add and improve log messages
Change mongoose logging to debug log level

Change development to verbose output

Image widget fix

See merge request !3

Users update

See merge request !2

Change nodejs to latest version

Add GPLv3 License

Closes #12

See merge request !1

Only temporary. Will be added again later on.

Allow empty widget name

Add owner validation

Removed adminLevel from users

Add missing simulator attribute in plot model

Simulations are the container for simulation models

Rename app.js back to server.js.

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.