Use /app to get app information instead of the app ID

This change mirrors how `/me` is used to get info for the current user instead of querying on the user's ID/slug. It reduces the information FaceplateSession needs to function (notably, this was its only dependency on the `plate` object).

I haven't tested this, but this may also fix heroku/facebook-template-nodejs/#11 with the code unpatched at 0d21bdb32609e8a44f576a40dd71434dcb9f0122 (if Facebook provides the app its own information through the `/app` route).