Fix broken Flickr Account list in Django Admin

Was getting an error while loading the Flickr Account list in Django
Admin. Ended in:

```
  File "/Users/phil/Projects/personal/django-ditto/devproject/venv/lib/python3.11/site-packages/django/contrib/admin/templatetags/admin_list.py", line 310, in __init__
    super().__init__(*items)
  File "/Users/phil/Projects/personal/django-ditto/devproject/venv/lib/python3.11/site-packages/django/contrib/admin/templatetags/admin_list.py", line 231, in items_for_result
    result_repr = display_for_value(value, empty_value_display, boolean)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/phil/Projects/personal/django-ditto/devproject/venv/lib/python3.11/site-packages/django/contrib/admin/utils.py", line 461, in display_for_value
    return _boolean_icon(value)
           ^^^^^^^^^^^^^^^^^^^^
  File "/Users/phil/Projects/personal/django-ditto/devproject/venv/lib/python3.11/site-packages/django/contrib/admin/templatetags/admin_list.py", line 182, in _boolean_icon
    "admin/img/icon-%s.svg" % {True: "yes", False: "no", None: "unknown"}[field_val]
                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
KeyError: '40fdeee308d1a21f'
```

It was because `ditto.flickr.models.Account.has_credentials()` was returning
a string, not a boolean.

The it turns out that the `self.assertTrue()` and `self.assertFalse()`
tests weren't too picky about whether something was a boolean or not.

Fixes #245

ditto/flickr/models.py

@@ -40,7 +40,7 @@ def get_absolute_url(self):
 
     def has_credentials(self):
         "Does this at least have something in its API fields? True or False"
-        return self.api_key and self.api_secret
+        return bool(self.api_key and self.api_secret)
 
 
 class TaggedPhoto(TaggedItemBase):

tests/flickr/test_models.py

@@ -30,11 +30,17 @@ def test_ordering(self):
         self.assertEqual(accounts[0].user.realname, "Bob")
         self.assertEqual(accounts[1].user.realname, "Terry")
 
-    def test_has_credentials(self):
-        account_1 = AccountFactory(api_key="1234", api_secret="9876")
-        self.assertTrue(account_1.has_credentials())
-        account_2 = AccountFactory(api_key="", api_secret="")
-        self.assertFalse(account_2.has_credentials())
+    def test_has_credentials_both_set(self):
+        account = AccountFactory(api_key="1234", api_secret="9876")
+        self.assertEqual(account.has_credentials(), True)
+
+    def test_has_credentials_neither_set(self):
+        account = AccountFactory(api_key="", api_secret="")
+        self.assertEqual(account.has_credentials(), False)
+
+    def test_has_credentials_one_set(self):
+        account = AccountFactory(api_key="", api_secret="9876")
+        self.assertEqual(account.has_credentials(), False)
 
     def test_get_absolute_url_no_user(self):
         account = AccountFactory(user=None)