R1:541d794c13df

Give ExternalAccount a providerConfigPHID, tying it to a particular provider

Summary:
Depends on D20111. Ref T6703. Currently, each ExternalAccount row is tied to a provider by `providerType` + `providerDomain`. This effectively prevents multiple providers of the same type, since, e.g., two LDAP providers may be on different ports on the same domain. The `domain` also isn't really a useful idea anyway because you can move which hostname an LDAP server is on, and LDAP actually uses the value `self` in all…