Make partner events optin

Ghost User
2018-10-29T15:54:11+01:00

### One-sentence description
Only show partner events to members who have the optin checkbox checked.

### Why?
Like the optin mails, not every member wants to see partner related content.

### Current implementation
The partner events are shown to everyone in the calendar.

### Desired implementation
Change the optin checkbox from optin for mailing to mailing and partner events.