User Stories:

• As an experiment owner, I want to extend my experiment's duration to increase our sample size, so that we can be more confident in our decision-making.

• As a designer or experiment owner, I want to extend my experiment's duration to ensure a consistent user experience for the treatment group (between experiment end date and eventual ship date) because we're making the decision to ship (i.e. the treatment has "won").

Right now, Experiment end dates can be changed on active experiments by updating the database directly. However, that's risky and requires experiment platform teammates to do so manually.

Acceptance Criteria
Let's relax our validation rules to:

• Allow end date to be changed on an experiment that starts in the future.
• Allow end date to be changed on an experiment that has already started.