Enable experimenters to edit active experiments

[cjqian]

Often, I'll start an experiment, preview as someone, then realize an issue / edit I'd like to make on the experiment.

Some fixes, related to #824 :

1. "Save" could be "Save as new experiment"
2. Or, "! Save (but delete current cohorts)"

[vivtsai]

Another way to solve this pain point would be to add a preview to the experiment editor (as it sounds like the root issue here is not being able to see what the experiment looks like without creating a cohort and adding a participant).