Pow in an umbrella project

Adding Pow to an umbrella project is near identical to a single app setup.

Instead of running mix pow.install, you should run mix pow.ecto.install inside your ecto app, and mix pow.phoenix.install inside your phoenix app(s).

You can follow the rest of the README instructions with the following caveats in mind:

• Run all mix ecto tasks inside the ecto app
• Run all mix phoenix tasks inside the phoenix app(s)
• For a project generated with mix phx.new --umbrella use :my_app_web instead of :my_app