Scheduled reminders: Clarify "Limit to" and "Also include" UI to prevent user errors

1. Add an empty option to the limit_to form field - and have that be the default for new reminders for all entities except Activities. The field is not a required field for Events, Memberships, Contacts - so in those cases the default should be <option value="">- neither -</option>

2. For all entities except Activities, change the label for that field to 'Limit or Add Recipients'. Hide #recipient select field if limit_to value is empty.

3. Add form_rule to validate that user has selected > 0 recipients (manual or Group ID), or participant roles if "Limit to" or "Also include" are selected. Form rule error message is:

"If "Also include" or "Limit to" are selected, you must specify at least one recipient."

4. Change DB default for limit_to from 1 to NULL (schema and upgrade)