[Feature request] Limit the number of appointments per attendee

[BarBaar44]

First of all: great app, I am using it very happily!

I have one call each month with my clients. Sometimes a client creates more than one appointment per month. I would like to limit this. Would be great if the app checks against the number of appointments already scheduled for this month.

So to make it generic: be able to set a maximum number of appointments during a specific time frame. In my case; only allow one appointment each month.

This can be validated using the used e-mail address.