Create a system for configurable emails in Symfony Mailer

Problem/Motivation

When building emails, there is often configurable data

1. required field, for example user emails require configuring body and subject; contact form emails require configuring to address
2. optional extra fields can be configured on any email: addresses, other headers, user language, theme, mailer transport, plain text alternative, etc

In the old mailsystem, each module created its own config form with underlying configuration data. Typically only a few select fields were available. We can improve on that with a single shared mechanism available for use by any module for configurable emails.

Proposed resolution

• @EmailAdjuster plug-in for each field that can be configured
• MailerPolicy config entity carrying the set of configured fields, indexed by email ID
• GUI for editing policy: page listing of all policy leading to policy add/edit forms
• Embedding policy editing: when editing the settings of a module or of a config entity, a GUI appears for editing the related policy

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet