travel: simplify travel information page

We don't care too much about where people arranging their own
accommodation are staying.  Keep the name to check expense claims but
get rid of the other fields.  Provide a room type field for people whose
accommodation we are arranging.  Hardcode the possible values for now.

While here, suggest that submitters don't waste their time entering
travel information until they've been told to do so (i.e.: when one of
their proposals has been accepted).