Add replay argument to service startup

The replay argument is only used by the ContestWebServer. This disables the
ip_lock mechanism and also tells the ContestWebServer to assume the contest
is always running.
Support for submitting multiple files at once

Added SubmitMultipartRequest class. This class needs to be passed in
an appropriate "files" list full of either [field_name,file_name] tuples
or [field_name, file_name_to_submit, file_to_read_from] triples. In the
triples, note that the last two fields can differ.