Guest booking

[NagariaHussain]

• Ability to "allow guest" checkout/booking
• This should be configurable per event
• When enabled, login is not required to book tickets/register for that event (although they could be)
• A user should be silently created with the provide email ID (don't send welcome email)
• Allow guests needed to be added in only the required APIs with proper perm checks if the checkbox is disabled
• Add unit and UI tests for this flow

[NagariaHussain]

@UmakanthKaspa