Replace all user-facing strings with references to lang files

[RogerDodger]

For example, instead of:

$c->detach('/forbidden', [ "You are not the admin" ]);

Use:

$c->detach('/forbidden', [ $c->string('notAdmin') ]);

Then, in lang/en/strings.yml:

notAdmin: You are not the admin

Once this is comprehensively done throughout the code, it will be possible to add support for other languages.