Use OpaqueEnvelopes for all passwords in Phabricator

Summary:
See D2991 / T1526. Two major changes here:

• PHP just straight-up logs passwords on ldap_bind() failures. Suppress that with "@" and keep them out of DarkConsole by enabling discard mode.
• Use PhutilOpaqueEnvelope whenever we send a password into a call stack.

Test Plan:

• Created a new account.
• Reset password.
• Changed password.
• Logged in with valid password.
• Tried to login with bad password.
• Changed password via accountadmin.
• Hit various LDAP errors and made sure nothing appears in the logs.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2993