Saving event crashes when registration start is placed before current date

Ghost User
2020-03-04T21:13:44+01:00

### One-sentence description Saving event crashes when registration start is placed before current date ### Current behaviour / Reproducing the bug 1. Create an event 2. Set registration start date in the future 3. Save event 4. Change registration date before current date 5. Crash (locally I get a `Fatal Python error: Cannot recover from stack overflow.`, which looks like infinite recursion ### Expected behaviour Setting registration start date before current date opens registrations