add new nbextension - copy shareable link

src/jupyter_contrib_nbextensions/nbextensions/share_link/README.md

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+ShareLink
+=========
+This extension adds a toolbar button to copy the shareable link to another user that have access to the notebook.
+
+![printview toolbar button](share-link-button.png)
+
+link preview: https://\<URL>/hub/user-redirect/notebooks/shared_folder/Untitled.ipynb?kernel_name=python3

src/jupyter_contrib_nbextensions/nbextensions/share_link/main.js

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+define([
+    'base/js/namespace'
+], function(
+    Jupyter
+) {
+    function load_ipython_extension() {
+
+        var handler = function () {
+            var curUrl = window.location.href;
+            var rgx = curUrl.match("\/user\/.*?(?=\/)");
+            var editedUrl = curUrl.replace(rgx,"/hub/user-redirect");
+            console.log(editedUrl)
+            navigator.clipboard.writeText(editedUrl)
+                .then(() => alert('the URL have been copied to clipboard'));
+        };
+
+        var action = {
+            icon    : 'fa-share-alt', // a font-awesome class used on buttons, etc
+            help    : 'Copy Shareable Link to Clipboard',
+            handler : handler
+        };
+        var prefix = 'shareLink';
+        var action_name = 'share-link';
+
+        var full_action_name = Jupyter.actions.register(action, action_name, prefix); // returns 'custom_extension:share-link'
+        Jupyter.toolbar.add_buttons_group([full_action_name]);
+    }
+
+    return {
+        load_ipython_extension: load_ipython_extension
+    };
+});

src/jupyter_contrib_nbextensions/nbextensions/share_link/share-link.yml

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+Type: Jupyter Notebook Extension
+Name: Share Link
+Link: README.md
+Description: "A little extension to copy shareable link"
+Main: main.js
+Compatibility: 4.x, 5.x