Improve handling of multi-leg and return tickets in Oui.sncf summary emails

For multi-leg journeys all involved trains are listed here, but neither
locations nor times of the involved connections. Since we cannot identify
all elements of journey correctly then, don't pretend we do that and ignore
the train number in that case. This also avoids merge problems when more
details become available later by other means.

Return journeys were not handled at all yet, we can use the same logic for
that with just different XPath search patterns.

BUG: 436976
FIXED-IN: 21.04.2