Fix regression where asynchronous creation of MDC events broke manual joint MDC events

This MR fixes a regression caused by e39e9c7c that causes https://git.ligo.org/emfollow/gwcelery/-/blob/0566a6e5be2cbb3b5cc9769757224572c3d862d7/gwcelery/views.py#L355-364 to fail. These changes now return a gpstime instead of a graceid in order to avoid a GraceDB call and re-enable the manual creation of joint MDC events.

This was tested using a gstlal test event:

which corresponds to https://gracedb-test.ligo.org/events/M349701/view/ (or one of the adjacent events).