Improve Share dialog (on mobile)

[ltguillaume]

Currently, after creating a poll, you get a share dialog:

• It is unclear that you can click the URL to copy it to the clipboard.
• The button next to the URL is to open it into a new tab. I don't see the use for this, plus I think the icon used for it is rather ambiguous.
• Instead, it should be easy to add a button to actually show the native share dialog of the OS (on mobile and even some desktop browsers), so that the link can be shared via every app supporting it: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/share. You can just check with if (navigator.canShare) if the button should be shown or not.