Add welcome page

Move the map from '/' to '/map' and have a welcome page with date/time pickers
and link to the other three pages as '/' page instead. When called without
'from' and 'to' parameters this welcome page picks '12h ago' and 'now' to
pre-populate those fields.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>