create_realm: Refactor to deal ASAP with key record, not string.

Now, there's just one spot at the beginning of the function where we
inspect the string key the user gave us; and after that point, we not
only have validated that string but in fact are working from our own
record that it pointed to, not the string itself.

This simplifies the code a bit, e.g. by not repeatedly searching the
database for the key (and hoping everything agrees so that we keep
getting the same row), and it will simplify adding logic to inspect
row attributes like `presume_email_valid`.

confirmation/models.py

@@ -132,13 +132,18 @@ _properties = {
 # Arguably RealmCreationKey should just be another ConfirmationObjT and we should
 # add another Confirmation.type for this; it's this way for historical reasons.
 
-def check_key_is_valid(creation_key: Text) -> bool:
-    if not RealmCreationKey.objects.filter(creation_key=creation_key).exists():
-        return False
-    time_elapsed = timezone_now() - RealmCreationKey.objects.get(creation_key=creation_key).date_created
+def validate_key(creation_key: Optional[str]) -> Optional['RealmCreationKey']:
+    """Get the record for this key, raising InvalidCreationKey if non-None but invalid."""
+    if creation_key is None:
+        return None
+    try:
+        key_record = RealmCreationKey.objects.get(creation_key=creation_key)
+    except RealmCreationKey.DoesNotExist:
+        raise RealmCreationKey.Invalid()
+    time_elapsed = timezone_now() - key_record.date_created
     if time_elapsed.total_seconds() > settings.REALM_CREATION_LINK_VALIDITY_DAYS * 24 * 3600:
-        return False
-    return True
+        raise RealmCreationKey.Invalid()
+    return key_record
 
 def generate_realm_creation_url(by_admin: bool=False) -> Text:
     key = generate_key()
@@ -157,3 +162,6 @@ class RealmCreationKey(models.Model):
     # True just if we should presume the email address the user enters
     # is theirs, and skip sending mail to it to confirm that.
     presume_email_valid = models.BooleanField(default=False)  # type: bool
+
+    class Invalid(Exception):
+        pass