Fix replacement of variables in subject

Problem/Motivation

Variable replacement occurs in SubjectEmailAdjuster::build().

• It's too soon - we haven't yet switched theme/language etc.
• It doesn't give any chance for hooks to alter the subject before token replacement
• If a hook need to replace variables in a subject, the code isn't reusable

Proposed resolution

1. Move replacement into Email::render()
2. Add an optional argument to EmailInterface::subject() to enable replacement

Remaining tasks

User interface changes

API changes

Data model changes