Add UserMailerTransformX and UserMailerSplitTo hooks

Add UserMailerTransformX and UserMailerSplitTo hooks

UserMailerTransformContent allows extensions to change the body of
an email sent via UserMailer::send(). This is applied before
low-level transformations such as multipart or content encoding.

UserMailerTransformMessage is similar but it is run after those
transformations.

UserMailerSplitTo allows extensions to request that a certain
user should always be emailed separately (so when UserMailer::send()
is called with an array of target addresses, that user will be split
out into a separate call). This is intended for content
transformations which need to be different per user, such as
encryption.

A side effect is that while before a call to UserMailer::send() was
either fully succeeded or fully failed, now the message might be
delivered to some targets but not others. send() will return a failed
Status object in those cases.

Bug: T12453
Change-Id: I4c3a018110173c3b5d52a753fdcbec397b590ced