Fix time zone bug in "Edit report" form

[ericpgreen2]

Before this PR, editing a scheduled report and clicking "Save" sometimes led the user to a 404 page. This happened because:

1. We instantiated the "Edit report" form with the time zone abbreviation, not with the underlying IANA time zone.
2. When the form was submitted with this time zone abbreviation, it resulted in an "invalid time zone" error in the virtual report file.
3. The presence of this error caused the report resource to be torn down.
4. Because the report resource no longer existed, a 404 page resulted.

This PR fixes this.

[djbarnwal]

Why not default to Etc/UTC here instead of empty string?

[ericpgreen2]

Good callout. All UI-created reports (which are the only ones editable via this dialog) will have a timeZone. Just updated to:

timeZone: reportSpec.refreshSchedule?.timeZone as string, // all UI-created reports have a timeZone