Trials documentation improvements

[wholmgren]

Distilled from my notes and @dplarson's notes on the second test trial.

• Further clarify the provided python script is meant as a simple example to help users get started and is not recommended for a real trial.
• Add warning that a password reset link is single use and cannot be reissued without breaking anonymity.
• Email with forecast links are nice. A simple copy/pasteable list of uuids would also be nice.
• Send email notifications when a trial is about to begin and when it has ended.
• Include an example of a probabilistic forecast in the documentation.
• Clarify wording around the forecast submission window.