Make it easier to copy the URL for a published note

### What

Make it easier to copy the URL for a note that has been published to the web.

### How

A couple suggestions:

* Include a "Copy" button next to the URL (like in the Electron app)
* Automatically copy the URL to the clipboard after publishing the note

Previously raised in https://github.com/Automattic/simplenote-macos/issues/29#issuecomment-369939494. Also raised in beta testing for v1.6 (h/t @anant1811, internal ref: p5T066-Xu-p2).

Start by locating the macOS interface that displays the URL for a published note, then compare its behavior with the Electron app's URL controls. Decide whether the finished change should provide a Copy button, copy automatically after publishing, or clarify the preferred behavior before implementation.

Written by the indexing model from the issue text.