🐛(backend) allow all printable ASCII in the user sub field

The user `sub` field was rejecting some ASCII characters that are
actually valid according to the OIDC spec.

Loosen the validation to accept the full ASCII range except control
characters, so the field is compliant with the RFC and works with
any spec-compliant identity provider.

Based on the Stack Overflow discussion in question 279832.

Closes #1609.

src/backend/core/models.py

@@ -27,6 +27,7 @@ from timezone_field import TimeZoneField
 
 from . import fields, utils
 from .recording.enums import FileExtension
+from .validators import sub_validator
 
 logger = getLogger(__name__)
 
@@ -145,19 +146,11 @@ class BaseModel(models.Model):
 class User(AbstractBaseUser, BaseModel, auth_models.PermissionsMixin):
     """User model to work with OIDC only authentication."""
 
-    sub_validator = validators.RegexValidator(
-        regex=r"^[\w.@+-]+\Z",
-        message=_(
-            "Enter a valid sub. This value may contain only letters, "
-            "numbers, and @/./+/-/_ characters."
-        ),
-    )
-
     sub = models.CharField(
         _("sub"),
         help_text=_(
             "Optional for pending users; required upon account activation. "
-            "255 characters or fewer. Letters, numbers, and @/./+/-/_ characters only."
+            "255 characters or fewer. Printable ASCII characters only."
         ),
         max_length=255,
         unique=True,