Allow empty widget name

Removed adminLevel from users

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.

Change response on delete routes.

Add UML class and use-case diagram (StarUML application).
Add admin user from config if no admin user found.

User passwords are stored with bcrypt hash & salt.
The authentication token is stored in the 'x-access-token' header.

For tests open mongodb port and access directly (without docker)