Adopt Redux reducer slice pattern

[carlgieringer]

Currently we have both:

• duplication between action and reducer definitions
• imprecise reducer typing (handleActions<EditorState, any, {requestPayload?: any}>) in at least our editor reducer maps.

If we adopt Redux's reducer slice pattern, we will both remove the duplication and get precise typing in the reducers.

We should also remove use of AnyAction.

[carlgieringer]

How to convert to a slice:

• Move actions into slice
  • Delete them from actions
  • Copy the prepare if necessary
• Replace usages of actions with slice's actions
• Search for usages of similar actions in sagas and move them into sagas adjacent to the slice.