Customize subscriptions in 1-click #3005

Try subscribing to this issue’s closure, it takes 4 clicks. Can this be reduced to 1 click?

Examples:

<img width="232" alt="group" src="https://user-images.githubusercontent.com/1402241/79323609-ba4b7b00-7f0e-11ea-8f1d-22b6298ac7c6.png" align=top> <img width="241" alt="icons" src="https://user-images.githubusercontent.com/1402241/79323631-c0415c00-7f0e-11ea-91ea-398f3896bd9d.png">

Or 3 clicks 😬 (but nearer/easier)

<img width="260" alt="" src="https://user-images.githubusercontent.com/1402241/79324444-ef0c0200-7f0f-11ea-9fbd-b6789dd976d0.png"> <img width="266" alt="" src="https://user-images.githubusercontent.com/1402241/79324446-efa49880-7f0f-11ea-808b-1c4d135f2290.png">

<details> <summary>Dropdown demo code</summary>

<details class="details-reset details-overlay" open="">
  <summary aria-haspopup="true" class="btn btn-block btn-sm thread-subscribe-button">
    {unmuteIcon()} Subscribe
  </summary>
  <div class="SelectMenu right-0">
    <div class="SelectMenu-modal">
      <header class="SelectMenu-header">
        <h3 class="SelectMenu-title">Customize</h3>
        <button class="SelectMenu-closeButton" type="button">
          {xIcon()} 
        </button>
      </header>
      <div class="SelectMenu-list">
        <button class="SelectMenu-item" role="menuitemcheckbox">
          {checkIcon()} Unsubscribed
        </button>
        <button class="SelectMenu-item" role="menuitemcheckbox">
          {checkIcon()} All notifications
        </button>
        <button class="SelectMenu-item" role="menuitemcheckbox">
          {checkIcon()} Merged
        </button>
        <button class="SelectMenu-item" role="menuitemcheckbox">
          {checkIcon()} Closed
        </button>
        <button class="SelectMenu-item" role="menuitemcheckbox">
          {checkIcon()} Reopened
        </button>
      </div>
    </div>
  </div>
</details>

</details>