new config var: Lab.config.homeForSharing

[#45204081], [#45204215], [#45204251]

Forces specific hostname for generating the sharing urls.

See description in readme:

http://lab.dev.concord.org/readme.html#project-configuration
http://lab.dev.concord.org/readme.html#interactive-share-link

Also updated text for Credits pane when sharing is disabled,
and added copyright info to credits pane.