You are already a participant of this event
#### Description
When joining an event as anonymous, no matter the e-mail, Mobilizon gives the error "You are already a participant of this event"
#### Steps to Reproduce
Having this in the config.

Opening event in incognito/private mode in the browser, pressing "Participate" and selecting "I don't have an account".
#### Expected behaviour:
That the anonymous user can join the event.
#### Actual behaviour:
Mobilizon gives the error "You are already a participant of this event".
#### Reproduces how often:
Every time.
#### Versions
* Mobilizon 3.0.0
* Waterfox G4.1.5
* Firefox Android
* Chrome Android
issue