feat(auth): use system browser for GitHub SSO / OAuth authentication

[setchy]

Closes #1764, #1761, #485

Changes

• register new custom protocol gitify://
• launch system browser for authentication flow
• create new OAuth2 app with new callback for development/testing

Backwards Compatibility
For backwards compatibility, the existing GitHub OAuth Apps for Gitify authentication will remain untouched, along with the callback handling at gitify.io/callback for a period of time.

Checklist

• update website FAQ docs for OAuth2 callback url - see docs: update for v6 release website#328
• resolve account mgmt render issue when adding/remove accounts
• create production OAuth2 app with new callback

[afonsojramos]

Thanks for looking into this! Exciting to see where v6 will take us :D