Make get_aspirants return None if no aspirants are found
Don't traverse up places through different parliamentary sessions

Place.get_candidates recurses up through the Place's parent
attribute to return candidates for enclosing areas.  However,
we have the slightly odd situation in Kenya where old
consituencies still have the new counties as parents, so
county+above level aspirants are still shown for old
constituencies.  This is confusing, so stop the recursion
up through the Place hierarchy if it would go from one
parliamentary session to a non-overlapping one.
Display a helpful message in place of aspirants on old constituencies

If no aspirants are found for a constituency, and this
constituency is for a past parliamentary session, we now
point this out, and provide links to the constituencies
that places that replaced this one.