Reinstate the code that allows us to search for existing games

As part of #64, arbitrator no longer searches for existing calendar events and modifies them. This is ok, since that code wasn't working incredibly well, anyway. However, we should either replace this with something that _does_ work (e.g. a warning before accepting a game that the user has a conflict for that time slot), or somehow be able to reinstate the old code that did (kind of) what we expected. See also http://stackoverflow.com/questions/40452022/why-does-the-google-calendar-api-get-stuck-when-called-from-within-a-promise for more information as to WHY we were stuck with having to disable this code.