Support native javascript share functionality

[Lorentz83]

Currently it is possible to enable buttons to share the feed item across various platforms (gmail, facebook, tubler...).

It would be nice to add support for native Javascript share() method (https://developer.mozilla.org/en-US/docs/Web/API/Navigator/share).

This integrates very well with Android (and I suppose IOS too) using the system's share UI. This allows to share the link with virtually any app installed.

As a bonus, if the current browser doesn't support or doesn't allow this functionality, it would be nice to fallback on a simple modal window which allows to copy the page URL.

Thanks!

[Athou]

Done!