Add a simple "admin UI" for managing tasks

[JeanMertz]

At first I wanted to focus on providing a UI for using Automaat, while using the GraphQL API for managing its state (during development, and in production).

But, given that the user-facing UI is in a pretty good state right now, and as development progresses, testing is slowed down by having to manually use the GraphQL API to set up certain states, I think it makes sense to introduce a very simple admin UI.

The current thinking:

• Add some kind of way to enable "editing mode" in the UI
• When on the home screen, in editing mode, there's a + sign somewhere to create a new task.
• Similarly, on the home screen, any tasks shown have a red X sign to delete tasks (with confirmation dialogue).
• When visiting a task, there's a way to switch to "editing mode" there as well.
• In this mode, you can change the title, description, and edit variables.
• This is the same view/mode you get into when you use the + sign to create a new task.

Most of this is already supported in the GraphQL API. Probably the most work will go into 1) implementing the UI elements, and 2) dynamically creating processor configurations based on the API types from GraphQL.

All of this is done without any support for authentication/authorization, so anyone can do this.

Eventually, this will be merged with #19 to limit who can activate this editing mode.