Fix bug in person conflict

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Move conflict check away from person model

All checks are done in form.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Move conflicting logic away from Model

Do everything inside ModeForm.full_clean()
This way we see all the time if someone has a conflicting reservation.
Plus we do not have to raise exceptions and fill their arguments.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Nested inlines must have a bug that requires this

During rendering it raises attribute error no cleaned_data exists.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
s/NEW/IGNORE in PersonForm

This semantically is better because it covers the modify path.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>