Reader: add native mobile sharing

In the app reader we have the native OS sharing available:

The [same API is available on browsers](https://developer.mozilla.org/en-US/docs/Web/API/Navigator/share), so when user is on mobile web, we could add the same action in the reader pretty easily under dropdown menu:

cc @lessbloat

Start in the app Reader's dropdown menu and review the browser Web Share API linked in the issue. Add the native sharing action for mobile web, matching the existing mobile app behavior, and verify that it appears and shares the current Reader item from the dropdown.

Written by the indexing model from the issue text.