R1:e46826ad364d

Introduce CAN_EDIT for ExternalAccount, and make CAN_VIEW more liberal

Summary:
Fixes T3732. Ref T1205. Ref T3116.

External accounts (like emails used as identities, Facebook accounts, LDAP accounts, etc.) are stored in "ExternalAccount" objects.

Currently, we have a very restrictive `CAN_VIEW` policy for ExternalAccounts, to add an extra layer of protection to make sure users can't use them in unintended ways. For example, it would be bad if a user could link their Phabricator account to a Facebook…