This PR was merged into the 4.4 branch.

Discussion
----------

[Mailer] Add support for multiple mailers

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | closes #32535
| License       | MIT
| Doc PR        | -

This adds the possibility to define several email transports.

If you only have one email transport,  nothing changes:

```yaml
framework:
    mailer:
        dsn: '%env(MAILER_DSN)%'
```

But if you need more than one, use the `transports` entry instead:

```yaml
framework:
    mailer:
        transports:
            main: '%env(MAILER_DSN)%'
            important: '%env(MAILER_DSN_IMPORTANT)%'
```

Then, when sending an email via the `Mailer`, pass the mailer name explicitly (by default, the first one is used):

```php
// use the first "main" transport
$mailer->send($email);

// or use the "important" one (`null` is the envelope)
$mailer->send($email, null, 'important');
```

The web profiler now displays the name and the shorten DSN.

Commits
-------

de5fae4 [Mailer] Add support for multiple mailers

This PR was squashed before being merged into the 4.4 branch (closes #12422).

Discussion
----------

[Mailer] Add support for multiple mailers

#12259 and symfony/symfony#33409

~~Do we need a `versionadded:: 4.4`?~~

![image](https://user-images.githubusercontent.com/1073881/66234058-05c5d680-e6ed-11e9-963f-d2a25c508aee.png)

Commits
-------

aeedbd2 [Mailer] Add support for multiple mailers