add HasSSM source property to VOEvents
@alexander.pace we will need a new source property field for the HasSSM source property in the VOEvents after emfollow/gwcelery!1473 (merged) is merged.
I closely created this MR in line with the other new field additions, but is missing the migration (maybe other pieces too?). If it is easy enough for you to add it, please do so. Also let me know of any other tests to add.
This new feature will require a future coordinated release of gracedb and gwcelery, but is not urgent.