ask user to log in if preauth url exists

[sethvincent]

This pr:

• introduces PREAUTH_NAME env var to be used with PREAUTH_URL
• if PREAUTH_URL exists the map will not render right away, instead a prompt will render asking the user to log in
• updates react-native-cookies with some small improvements
• adds a little documentation to the .env.sample file

[sethvincent]

This now implements the idea that the map shouldn't be rendered until after authorization and provides an explicit login button

It also introduces a PREAUTH_NAME env var so that we can configure the name of the auth service.

There's some more testing I'd like to do to make sure this still works in various testing/production environments, but otherwise is very close to ready.

[geohacker]

@sethvincent this looks really good to me. I tested it on both OSM production and dev on iOS and Android. Worked well 👍