FEATURE: Add banner prompt for user consent

[featheredtoast]

Add an option to prompt a banner for a user's consent on the main screen (off by default).

• Do not show the banner if a user has already subscribed, or has dismissed the banner.
• Do not show the banner if a user is logged out.
• Add a universal confirmation notification once a user subscribes, similar to slack/discord. This also can help confirm that the notifications are in good working order.

Most of the work here was inspired by the way slack handles notifications, by having a predominant prompt without nagging about it.

[ZogStriP]

Screenshots?

[featheredtoast]

Consent banner prompt:

Subscription confirmation notification:

Admin options:

[xfalcox]

When I click on the banner there is a post to the rails backend:

Started POST "/push_notifications/subscribe" for 1.2.3.4 at 2017-11-06 21:16:01 +0000                                            
Processing by DiscoursePushNotifications::PushController#subscribe as */*                                                              
  Parameters: {"subscription"=>{"endpoint"=>"https://updates.push.services.mozilla.com/wpush/v2/longcode", "keys"=>{"auth"=>"longkey", "p256dh"=>"evenlongerkey"}}, "send_confirmation"=>"true"}
Completed 200 OK in 782ms (Views: 0.5ms | ActiveRecord: 8.6ms) 

But I never get the confirmation notification. Tested on latest stable Chrome and nightly Firefox on Android.

[featheredtoast]

Oh dear, another case of the "it works on my machine"s -- I'll try and track this down, thanks for the feedback!

[featheredtoast]

I suspect you might need to update your service workers, too -- It could be possible that there's an old version lying around.
I hit this a few times where a service worker would need to be forcefully updated to receive messages.

At least for the chrome devtools, application -> service workers tab will allow you to forcefully re-register.

This specific issue with automatically registering new service workers should be dealt with, but I don't think that's necessarily in scope here.

[featheredtoast]

I'm able to reproduce your issue by changing the subscription back to "ask" and refreshing the page, and toggling the notifications back and forth again, but I get a TypeError: no implicit conversion of String into Integer

Logging out and back in seems to resolve it for me - I'll document that in another ticket though. If that was what was preventing the notification, it's unrelated to this.

[tgxworld]

Is there an opportunity to re-use {{discourse-banner}} instead?

[tgxworld]

I think we should extract the common code in Pusher into a method for this method to call.

[featheredtoast]

Alright, I'll fix this up and apply the reviews on this one when I've got some time!

[featheredtoast]

This PR is back in working order. A few notes:

After looking into it, {{discourse-banner}} seems specific to banner topics, and this is a per-user banner that will appear to notify users that push notifications are available, so it doesn't seem feasible to re-use the component.

I am planning to re-style the banner to support the same look+feel as the other notification before I merge this.