Error with mix admin.install on Phoenix 1.3

[Katherine Bowler]

This is for the phx-1.3 branch.

To reproduce:

• Add ex_admin to the project with {:ex_admin, github: "smpallen99/ex_admin", branch: "phx-1.3"}
• Add the following to your /config/config.exs:

config :ex_admin,
  repo: MyProject.Repo,
  module: MyProjectWeb,
  modules: [
    MyProjectWeb.ExAdmin.Dashboard,
  ]

• mix do deps.get, deps.compile
• mix admin.install

Returned error is:

** (RuntimeError) Could not find web path 'lib/app_web_web'.
    lib/mix/utils.ex:24: Mix.ExAdmin.Utils.web_path/0
    lib/mix/tasks/admin.install.ex:230: Mix.Tasks.Admin.Install.do_dashboard/1
    lib/mix/tasks/admin.install.ex:55: Mix.Tasks.Admin.Install.do_install/1
    (mix) lib/mix/task.ex:301: Mix.Task.run_task/3
    (mix) lib/mix/cli.ex:75: Mix.CLI.run_task/2

Note the 'lib/app_web_web' on the first line of the error.
In lib/mix/utils.ex:24 it appended an extra _web, however Mix.Phoenix.otp_app should already return this as app_web.

Started fix in #423, but needs more work.