Add route mixin for setting a hint per project create section

- use deactivate / activate functions on routes rather than private exit / enter

@jakoblaegdsmand To use: add the hint property to one of the Project create controllers, eg App.MyProjectPitchController:

    hint: function () {
      return gettext("This is a user hint");
    }.property(),

Then use the hint in the sidebar template as usual: {{hint}}.