Ensure GW candidates in `SubGRBTargeted` search pass threshold and are correct

The following discussion from !1301 (merged) should be addressed:

• @brandon.piotrzkowski started a discussion: (+1 comment)

  As mentioned in #693 (comment 790716), should we add a condition for the GW FAR for search='SubGRBTargeted' events or make that a separate MR?

• We should also consider adding some selection criteria similar to the superevent manager, to ensure the "best" superevent candidate is chosen when deciding between multiple superevents in to associate with an external event (and eventually figure out a longer term solution to switch external events between superevents).