Add a couple web pages to improve the onboarding experience

[ktbee]

Currently when you add thanksbot to a new workspace, the install success (or error) message is just a white page with some unstyled text (relevant code here). It would be nice to instead send back a nicely styled page that celebrates adding thanksbot and offers any next steps we'd like users to take (try out sending a DM?).

Likewise we don't currently have a homepage when you visit the app's URL on Heroku (https://thanksbot-app.herokuapp.com/). I don't know if this is something we want to optimize or leave not-so-public-facing, but it could be a good place to explain the project and also have a "Add to Slack" button.