Shows cannot be submitted without a presenter

[wlabarron]

The main issue with this is that shows selected from the dropdown fail without a clear error message if there is no presenter associated.

Possible solutions:

• Don't allow shows without presenters to be selected from the dropdown. Hiding or disabling the options would fix this but it could confuse admins when shows don't appear in the list.
• Allow presenter-less shows to be submitted. This would need work to ensure these shows still get saved somewhere sensible, e.g. providing default values for the variables in file names and show titles.