Unifying settings UI

[adekbadek]

Right now, the configuration might happen in four different places:

• Newspack Plugin - on setup
• Newspack Plugin - Engagement Wizard
• this plugin - native settings screen
• this plugin - when creating a newsletter while ESP settings are not set

This is not optimal, because any changes to the settings screen have to be synchronised among all these UIs. There should really be a single UI for the settings.

This is an opportunity to rethink how the Newspack Plugin interacts with the other plugins. One way to do it may be to use iframes in the Newspack Plugin, which would display the paricular plugin's UI.