"Report" button behaviour confusing when navigating to notebook for first time

Right now clicking the report just shows the report based on the state of the current set of evaluated cells, which is kind of counter-intuitive in the case that you're just navigating to a notebook for the first time. Basically you just wind up with a blank page and it's not clear why.

Possible changes we might want to make:

* Make report mode the default?
* If in editing mode and there are unevaluated cells, should we automatically run them when entering report mode? Or maybe at least prompt? Another alternative would be to open the report view in a new tab instead of making it a modal option. This is what glitch does.

Start by reproducing the first navigation to a notebook and clicking the Report button while noting the current evaluated-cell state. Review the report mode and its entry behavior, then clarify which proposed interaction is wanted; done means a first-time visitor is not shown a confusing blank report and the behavior is clear.

Written by the indexing model from the issue text.