Add constraint validator to prevent invalid recurrence patterns

Problem/Motivation

The recurrence fields don't currently validate whether the pattern you've entered makes sense.

Proposed resolution

Add a constraint validator that checks the recurrence pattern to confirm that it actually yields at least one instance before allowing the series to be saved.

User interface changes

Attempting to create a series with a nonsensical pattern will now result in a form error on the event series and prevent it from being saved until corrected. Enabling this new constraint should be optional in the event series settings.