Add new event type, pull-request-new, for logging incoming pull
requests.

This will allow CI systems to track PRs just as easily as regular
series.

The change also loosens up constraints imposed on EventLog model - it
used to assume always having an associated series, which is not true for
the pull requests. PRs are standalone patches.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>