[5.3] Add fluent email priority setter

[KKSzymanowski]

Regarding #15153

[KKSzymanowski]

One thing. I haven't found any tests for the Mailable class.
I didn't know where to put tests for this functionality so I didn't write any.

[GrahamCampbell]

Are there constants for these inside swiftmailer?

[KKSzymanowski]

I haven't found any.

This is the SwiftMailer method header:

/**
 * Set the priority of this message.
 *
 * The value is an integer where 1 is the highest priority and 5 is the lowest.
 *
 * @param int $priority
 *
 * @return Swift_Mime_SimpleMessage
 */
public function setPriority($priority);

[KKSzymanowski]

There are methods to and bcc on Illuminate\Mail\Mailer(so also on Mail facade). Should I add priority method to Illuminate\Mail\Mailer and to Illuminate\Mail\MailableMailer classes?

Also, why there's no cc method on Illuminate\Mail\Mailer?

[KKSzymanowski]

@taylorotwell, @GrahamCampbell
What about Mail facade? Should I add the priority(or withPriority) method there as well or leave it like it is?