Remove memoizer from UI

[jnunemaker]

Default rails and flipper setups will use memoizer so having ui include it causes the double memoize warning message, which can be confusing.

This removes the memoizer from UI and adds the backwards compatibility stuff we need to make it all work the same.

We can add a doc on how to add your own memoizer for just the ui:

run Flipper::UI.app { |builder|
  builder.use Flipper::Middleware::SetupEnv, some_flipper, env_key: "some_flipper"
  builder.use Flipper::Middleware::Memoizer, env_key: "some_flipper"
  # etc
}

We can also add a doc on how to add your own memoizer for just the api:

run Flipper::Api.app { |builder|
  builder.use Flipper::Middleware::SetupEnv, api_flipper, env_key: "api_flipper"
  builder.use Flipper::Middleware::Memoizer, env_key: "api_flipper"
  # etc
}

I'm open to doing something different. Just figured I'd throw out a first example to react to.

href #526

[jnunemaker]

@bkeepers updated. Is this more like what you were picturing?

[bkeepers]

Looks great!