Set account/langcode from recipient

Problem/Motivation

For code that is sending email to a user then we'd like a quick way of setting the recipient, langcode and account for rendering.

If sending email to a configured address that happens to correspond to a user then we should do the same. For example, the address for automatic update messages is likely to match a user account.

Proposed resolution

• Enhance Email::setAccount() function to set recipient and langcode if they are not already set.
• Allow passing NULL to Email::setAccount() to default the account from the recipient.

Remaining tasks

User interface changes

API changes

Data model changes