Add a notification foldout

It would be great to have a notification icon in the corner of the app, just like on the website.

Notifications API

When switching to a repo connected to GitHub (dotcom or Enterprise):

1. If the app has fetched notifications for the current account Recently™, pull the list of notifications from a cache and display them.
2. Enqueue a fetch for the current account.

When a fetch is enqueued for an account:

1. If a fetch has already been enqueued for the account, return without doing anything.
2. If the next-fetch date stored for the account has passed, fetch for the account immediately.
3. If the next-fetch date has not passed, wait until it has passed, then fetch for the account immediately.

When fetching for an account immediately,

1. GET the /notifications endpoint, passing the If-Modified-Since header with the last-modified date saved in the database for the account.
2. Set the next-fetch date for the account to Date.now() + Number(headers['X-Poll-Interval']) * 1000, where headers contains the HTTP response headers.
3. Set the last-modified date for the account to the value of the Last-Modified header.
4. Update the list of notifications.

[nesl247]

@billygriffin Hopefully you do reconsider at some point, or get the web team to implement push notifications. This is really the big issue.

[billygriffin]

Thanks for the request here @j-f1, I totally understand how this could be useful. However, we’d like to think about notifications contextually in the course of various workflows. For example, the notification when diverging from default branch is intended to ensure you have the latest from master on your branch. So as we continue to build out functionality please let us know where you see opportunity to potentially integrate contextual notifications, but I don’t think a full port of GitHub notifications into Desktop is going to add a whole lot for folks.