Opt-in Email Notifications (including build and artifact / app is ready to install email) instead of opt-out one, with more granularity

Description of the feature request

Change the current opt-out system to an opt-in one.

Right now you can opt-out of email notifications for a specific app by clicking the Watch/Unwatch button on the app’s bitrise.io page.

The issue is:

  1. It’s an opt-out system not an opt-in one, which is quite painful if you have a larger team creating new bitrise.io apps all the time which you have to opt-out from one by one.
  2. Lack of granularity
    • Right now if you “Unwatch” an app on bitrise.io that will prevent every kind of email to be sent. Ideally you should be able to decide separately whether you want to receive Build Status emails as well as for Artifact/App is ready for install email.
    • Additionally, ideally it shouldn’t just be an on/off switch, in case of Build Status emails I should be able to decide whether I want to get emails about successful builds or just about failed ones, or only the ones where I participated in the related PR for example. So, basically what you can set on the Settings tab of the app right now, but on a personal / per-account level instead of on a per-app level.

Use case / for what or how I would use it

Minimizing email notifications.