Implement translation on backend

[tobyzerner]

Currently all text output by the backend (validation messages, emails, etc.) is hardcoded in English.

What needs to be done:

• Make the Translator class adhere to Symfony's Translator interface. This is so that it can be used with Laravel's validation component.
• Add translations for Laravel's validation component messages.
• Instantiate the Translator instance in LocaleServiceProvider using the actor's preferred locale.
• Make emails translatable.

[franzliedke]

We also need to make the YAML parser resolve references, right?
Probably a good place for me to start adding some more unit tests.

[tobyzerner]

Yep, good idea. Though should we resolve the references in the YAML parser, or in the Translator (front + backend)? The former would be more DRY, the latter might save some bytes on the compiled JSON?

[franzliedke]

I think I'd like to have the references already resolved in the compiled version... so that means do it in the parser, right?
(And I think this makes sense, since it really is a feature of our specific YAML syntax.)

[tobyzerner]

Yep ok, makes sense.

[tobyzerner]

@dcsjapan How's something like this for the email translation keys:

core:
  email_activate_account_subject: Activate Your New Account
  email_activate_account_body: |
    Someone (hopefully you!) has signed up to {forumTitle} with this email address.

    If this was you, simply click the following link and your account will be activated:
    {url}

    If you did not sign up, please ignore this email.

(Read this if you're not familiar with the | syntax)