Simplify Actor Modes

Actor modes is one of the most complex concepts new users are exposed to and expected to make a decision on when they first activate the plugin.

Let's find a way to simplify this and ideally remove the need to make this a decision point. We could default the Blog Actor to always be active in the background and rely on the `activitypub` capability management in the user screen to create individual actor profiles.

For this to work, we'd also want to update the welcome screen checklist and consider adding an educational piece for existing installations after updating the plugin.

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