Constrain Editing of existing Ensembles

[tedyoung]

For future ensembles: only allow editing of Name, Zoom link, and date/time.

For past (but not completed) ensembles: only allow editing of Name

For completed ensembles: edit only video recording link

[tedyoung]

From #24:

Name: may not allow edits after Completed

Start Date/Time changes not allowed:

New Date/Time in the past For events that are in the past -- must use Reschedule For Completed