Place public urls if you plan to use Attendize as a backend service intended for internal usage, with a separate frontend

[quentincaffeino]

I'm not sure how much is this an issue for other users of this repo. But in my opinion frontend is the face of the business so we develop custom frontend for our. And we encountered the issue that all the urls are built with the url from request object. Which is a problem when:

1. You host your frontend and backend on different domains (subdomains in my case)
2. If you access your backend using internal docker domains (example http://attendize/)

The most important part of this PR is two helpers: public_route & app_url.

First function exists to produce links to public urls. For example a link to download tickets will be opened on custom frontend.

Second was created to put absolute links to assets in emails. Because if you trigger email generation from private docker network all links would be inaccessible from public network.

Both helpers work by basically always forcing specific domain. For public_route it is a domain set in PUBLIC_URL env variable. For app_url it always forces APP_URL env var.

Both will still generate regular url if PUBLIC_URL was not set.