Installation wizard should auto-populate github id/secret on heroku button

[timwis]

From @pezholio in #55:

You can also add params to the deploy button URL in the format env[OAUTH_CLIENT_ID]=whatever and env[OAUTH_CLIENT_SECRET]=whatever, which might remove some potential pain points, so all people have to do is enter their app name. Would also mean if someone accidentally closed the tab with their app details they won't have to go hunting around for the secret

Clever idea, should be a pretty easy lift. Just wanted to break it out into its own feature in order to move the rest forward.