Make graceid a Python property for events

docs/admin_docs/source/miscellaneous.rst

gracedb/alerts/email.py

gracedb/alerts/phone.py

gracedb/alerts/xmpp.py

gracedb/api/v1/events/views.py

gracedb/api/v1/superevents/serializers.py

gracedb/api/v1/superevents/tests/test_access.py

gracedb/events/admin.py

gracedb/events/buildVOEvent.py

gracedb/events/feeds.py

gracedb/events/models.py

gracedb/events/tests/test_perms.py

gracedb/events/view_logic.py

gracedb/events/view_utils.py

gracedb/events/views.py

gracedb/search/utils.py

gracedb/superevents/utils.py