Backend code refactoring / cleanup

[Ferdinand Mütsch]

• Better modularity
  • Currently, most logic is contained inside controllers, which also define the REST interface at the same time. Business logic and presentation logic are strongly interleaved, which makes it super hard to be reused. Some parts methods are outsourced to controllers/utils in a quite hacky way and without following a common interface.
• Refactor to modern ES6-style JavaScript syntax (requires #53)