Send intake form invitation emails to participants. If participant_ids is not provided, sends to all participants who haven't received the form yet.

Supported merge tags in subject and body:

• {{first_name}} - Participant's first name
• {{last_name}} - Participant's last name
• {{full_name}} - Participant's full name
• {{email}} - Participant's email
• {{cohort_name}} - Cohort name
• {{form_link}} or {{intake_form_link}} - Unique intake form URL