Complete mediator implementation

[pkraker]

Before we progress with improvements on the frontend, we should complete the mediator implementation. The goal is that there are no cross-module calls any more. All logic between modules should be handled by the mediator.

[Bubblbu]

Progress

PR #96:

• Delegates many calls from papers/bubbles/list to mediator;
• Adds render_list and render_bubbles option to default-config.js
• Mediator now only calls papers/bubbles/list if configured.
• Moves config from headstart to config
• headstart.js was simplified