This Fixes #1038 : use tag for shareable link

FYI, this syntax actually doesn't sequence `update_link` after the preceding part.  `x.then(y)` evaluates x and y, and then schedules the value of y to run after x has finished.  So this will run `tag_notebook_version`, then create the `function(ret)`, then run `update_link`, and then sequence the result of each to wait on the last one.  What you want is `.then(function() { RCloud.UI.share_button.update_link()); });`