Introduction of 'offline' parameter

Adds a boolean parameter to the createEvent() method which specifies whether the event was found by an offline (True) or online/low-latency search (False). Default is set to offline=False so that in the worst-case scenario, an offline event is marked as online, which is the same as the current behavior.

Note that this branch is based off of !15 (merged), so that should be merged first (hence why this is marked as WIP). Currently the changes shown below feature those changes as well. This requires a server code update as well, which is shown here: gracedb!4.