Use UTC internally

As per commit d0a253351dc89d7e8ecae9ac147be4cd5f69d05a of srcomp, this
changes all uses of 'datetime.now()' to 'datetime.utcnow()' to make sure
the dates used internally are UTC.
Return an error object if the match is None
Remove the knocout/max_entrants endpoint

I'd completely forgotten that _all_ the teams are in the knockouts,
not just a subset of them. As a result, this endpoint is pointless.

Change-Id: I38e55af114453cbb50ea4462761577d3de7fa7f8