Horizon Server should support authentication from other host

[dalanmiller]

When serving Horizon app files statically from another host, oauth currently will not work as there is no mechanism for Horizon Server to redirect you back to the static file host following a successful/unsuccessful oauth authentication attempt.

@Tryneus suggests having a variable in the Horizon Server config file which is the explicit host and path which the server should redirect to following the authentication attempt.

An example could(?) work like this:

On the static file server - app.horizon.internal-cloud.com

Since one might have multiple, generated, or might not have easily human-readable host names, the Horizon Client API should have some sort of short hand like this:

var horizon = Horizon({host: "server.horizon.internal-cloud.com:37182", authType: "token"});
// ...
// User clicks on Twitter Login Button

// This redirects browser to "server.horizon.internal-cloud.com:37182/horizon/twitter" 
horizon.auth("twitter").redirect() 

On Horizon Server - server.horizon.internal-cloud.com

• Go through typical oauth process
• Concluding (un)successful authentication, Horizon Server checks configuration file and redirects them back to app.horizon.internal-cloud.com?horizon-jwt=.....

Back on static file server - app.horizon.internal-cloud.com
The Horizon Client Library sees the query params and properly stores the horizon_jwt in localStorage. (current and complete functionality)

[deontologician]

@dalanmiller @Tryneus is the auth_redirect property the fix for this?

[deontologician]

Needs verification this doesn't currently work