Add subject_email parameter

[sdespont]

This PR brings the possibility to define the subject of the mail sent with the authentication code.
The parameter subject_email have been added, with a default value equals to the actual text "Authentication Code"

[sdespont]

@scheb what do you think about having a configurable prefix text before the code in the email? I don't want to spend time to push a PR if you don't agree with this approach

[scheb]

Hmm, wouldn't want to add more configuration options. I'm not having the intention to provide the ultimate configurable mailer with the bundle.

Way to go would rather be to implement a custom mailer, which generates the email exactly as you want it. The on-board mailer is just doing the bare minimum to have something working.

[sdespont]

@scheb you are right, implement a custom mailer is easy https://symfony.com/bundles/SchebTwoFactorBundle/current/providers/email.html#custom-mailer

After reflection, suffix or prefix is not a good idea. But permitting to set the email subject of the default email still makes sense for me.

[scheb]

There seem a whole lot of changes in the diff, that aren't related to this merge request. In its current form I cannot merge it, as it will mess things up.

If you want to proceed, please review the diff and provide a cleaned-up changeset that just contains your changes.

[scheb]

Closing due to inactivity. If you feel like follow-up on this, please create another PR. Thank you!