Move to JWT-based auth

[elimisteve]

Instructive examples:

https://godoc.org/github.com/dgrijalva/jwt-go#example-NewWithClaims--CustomClaimsType

https://auth0.com/blog/authentication-in-golang/

https://dinosaurscode.xyz/go/2016/06/17/golang-jwt-authentication/

http://www.giantflyingsaucer.com/blog/?p=5994

[elimisteve]

Should only require backend changes, awesomely.

This is an important step in being able to expose PostgREST to the frontend.