Add in ability to search on Participant Registration Date

I have had a user in my organisation wanting to search based on when people have registered for events. This was basically about trying to measure engagement. I figure that this would be a nice improvement to The Event form search ability.

I have put the fix version as 4.8 as i believe 4.7 is currently in a feature freeze, However i expect that this should be able to be backported to 4.7 and also 4.6

Seamus