[finishes #73238226]: enable re-sharing of modified shared levels

Related to https://github.com/code-dot-org/dashboard/pull/822

Previously, we wouldn't let the user "report" a program to the service if they were viewing shared program (/sh/<num> URL). Additionally, the explicit "share" button was never displayed for bounce/flappy/studio when on a "share page".

Now that the dashboard can handle this scenario by providing a useful callback URL to report data to, I removed those restrictions in the blockly apps.

The net result is that you can "mod" other people's programs and share them again. And you can also use this as a simple save feature when you are working on a complex program. Each time you share, it gives you a new link to use when you want to continue working later.
Couldn't we just call onAttempt(report) in this case instead of calling onAttemptCallback()? At the very least, I'd prefer to make the fact that builderDetails is null more explicit by calling onAttemptCallback(null).