sharing fixes

* don't set minWorkspaceHeight when there is no block workspace (switch on config.hide_source instead of BlocklyApps.noPadding)
* don't show Print button on main page (event handler wasn't hooked up - Andre may add this back later)
* do show the templates/learn.html blurb on mobile shares - it is essentially the same as the config.makeYourOwn blurb that appears for all apps other than artist
* fix the code that generates links in the learn.html blurb to generate proper code.org URLs from either studio. or learn. addresses (previously, it only removed "learn." from the host name)