Make "mailer" service optional

[covex-nn]

With this PR the mailer service will not be required any more while compiling Container. Both mailers will throw exceptions on sending email if mailer service was not available.

Also a compiler pass CheckForMailerPass, introduced in #2639 was deprecated, and i created a new class TwigMailer without type hint in first argument of constructor. Old TwigSwiftMailer was deprecated, i moved all properties and methods to a new class to avoid BC breaks.

This PR with #2708 will make creating a recipe for Symfony Flex easier

[devtronic]

$mailer Should be typed as \Swift_Mailer

[covex-nn]

No it should not =) With a type hint it would not be possible to pass null value. Old TwigSwiftMailer had that type hint. I could not remove it from there because it would be BC break. So - i created a new class TwigMailer, a parent for old TwigSwiftMailer

[devtronic]

Ok, what's about

if(null !== $mailer && !($mailer instanceof \Swift_Mailer)
{
    // throw ....
}

?

Currently you can pass anything as $mailer... :D

[covex-nn]

It turned out that there are more then one mailer! I found this out in @stof's comment last year in #2562 (comment) =) That is a reason why symfony/swiftmailer-bundle is not required dependency, btw. So, it is not necessary, i guess

[devtronic]

I think that there is no another mailer which uses the Swift_Message class (which comes from the swiftmailer package)
https://github.com/covex-nn/FOSUserBundle/blob/b3000d0088536a1384b7057aa0888a9caa42a45e/Mailer/TwigMailer.php#L115

[covex-nn]

@devtronic actualy, i agree with you, and i am on your side! May be symfony/swiftmailer-bundle should be in require section of composer.json! Create a PR and i will be very glad if it will be accepted, then i will close this one =)

But, Swift_Message comes from swiftmailer/swiftmailer and it is not a bundle. So, it is possible to create a bundle, that provides mailer service and uses Swift_Message as a first argument of send method at the same time.

[covex-nn]

A travis job with COMPOSER_FLAGS="--prefer-lowest" failed, because it took a lot of time for composer update. Please, restart that job

[stof]

Closing for the same reason than #2915

#3042 contains a change to avoid requiring the mailer service when not needed