Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
@cvogt We talked about making the `Effects` here local to the DB, not just the Backend. After working on the unit tests and sample code, I think we should drop them entirely. While action effects can be inferred very nicely, it's hard to write down types for Actions. We could still create and propagate the effects but not enforce them when executing an Action. The `Action` type would be renamed to sth. like `EffectfulAction` which carries the effect type. All APIs would produce such actions. The standard `Action` type would be a type alias for actions with all possible effects. That way users would not be burdened by effect types unless they are actively using them to restrict effects in their own code. The only remaining downside (compared to removing them entirely) is that they still show up in error messages (where you'd always see EffectfulAction, not Action)