EditSession page should provide better experience for editing start and end time

[DamianEdwards]

This is the current experience for editing the start/end time on the EditSession page:

It currently relies on the default input field rendering for DateTimeOffset which isn't great, due to issues with the browsers' own pickers not supporting round-trippable formats.

We might have to split the date/time and time-zone offset info into their own input fields with the date/time portion being one, and a time-zone picker being the other, e.g.:

<input name="StartTime.DateTime" type="datetime-local" />
<select name="StartTime.Offset"> ...