R1:e2919211f3c2

Avoid PhabricatorApplicationTransactionStructureException on editors not supporting Mute Notifications

Summary:
Do not expose the "Mute Notifications" sidebar menu entry when the underlying Editor for that object type does not support muting notifications (means: creating a transaction of type "core:edge") at all.
This avoids a disappointing `PhabricatorApplicationTransactionStructureException` after two clicks.

Disabling the menu entry while still exposing it makes no sense here as the user could never…