Allow "MFA Required" objects to be edited without MFA if the edit is only creating inverse edges
This includes some ambiguous cases like "unsubscribe", but also includes "mention", which seems like clearly bad behavior.
Allow an "MFA" object to be the target of mentions, "edit child tasks", etc.
- Mentioned an MFA object elsewhere (no MFA prompt).
- Made an MFA object a subtask of a non-MFA object (no MFA prompt).
- Tried to edit an MFA object normally (still got an MFA prompt).
Reviewed By: amckinley
Maniphest Tasks: T13222
Differential Revision: https://secure.phabricator.com/D19901