[META] Use Policy GUI even with back-compatibility

Problem/Motivation

This module has policy config and GUI that allows customising outgoing mails precisely. We can add many headers, with lots of flexible options. Filters can apply with a very specific granularity for a certain type of mail, or a single filter can apply widely saving duplication.

Various contrib modules also do the same in a more limited way: only some headers and each maybe in a limited way. Example modules:

• Contact can set to.
• Simplenews can set from, priority, email receipt.
• Webform can set to, from, cc, bcc, reply, etc
• Commerce can set bcc and from

We can use the Symfony Mailer policy throughout, reducing code duplication and create a consistent admin experience. This can work even in "Proxy Mode", when the EmailBuilder is part of this module.

Proposed resolution

General framework:

1. #3262882: Policy importing
2. #3272683: Policy importing GUI
3. #3262883: Extend address policy elements
4. #3270172: Address policy should support multiple addresses
5. #3263870: Add view/edit of related policy to existing config/settings forms
6. #3263447: Extend hook_mailer_bc_alter() to allow altering 'to' and 'reply-to' (we would ignore the passed value if we are using policy instead).

For each module #3265253: Add policy listings to module entity/settings forms:

1. Alter the module configuration form (for the contact form, newsletter, etc) to remove the settings that duplicate policy and instead provide a link to edit the Symfony Mailer policy.
2. Remove code from the EmailBuilder relating to these settings/headers.
3. Write a policy import function to read the old settings and write corresponding policy.

Remaining tasks

User interface changes

API changes

Data model changes