Make constance configs usable in other settings

Introduces a utility function lazy_config() that can lazily bind
a constance config key. As an example, it is used to use the primary colour
configured through constance as the app's theme colour.

Advances #139

aleksis/core/settings.py

@@ -7,7 +7,7 @@ from django.utils.translation import gettext_lazy as _
 from dynaconf import LazySettings
 from easy_thumbnails.conf import Settings as thumbnail_settings
 
-from .util.core_helpers import get_app_packages, merge_app_settings
+from .util.core_helpers import get_app_packages, lazy_config, merge_app_settings
 
 ENVVAR_PREFIX_FOR_DYNACONF = "ALEKSIS"
 DIRS_FOR_DYNACONF = ["/etc/aleksis"]
@@ -368,7 +368,7 @@ _settings.populate_obj(sys.modules[__name__])
 
 PWA_APP_NAME = "AlekSIS"  # dbsettings
 PWA_APP_DESCRIPTION = "AlekSIS – The free school information system"  # dbsettings
-PWA_APP_THEME_COLOR = _settings.get("pwa.color", "#da1f3d")  # dbsettings
+PWA_APP_THEME_COLOR = lazy_config("COLOUR_PRIMARY")
 PWA_APP_BACKGROUND_COLOR = "#ffffff"
 PWA_APP_DISPLAY = "standalone"
 PWA_APP_SCOPE = "/"

aleksis/core/util/core_helpers.py

 import pkgutil
 from importlib import import_module
-from typing import Sequence, Union
+from typing import Any, Callable, Sequence, Union
 
 from django.conf import settings
 from django.db.models import Model
 from django.http import HttpRequest
+from django.utils.functional import lazy
 
 
 def dt_show_toolbar(request: HttpRequest) -> bool:
@@ -67,6 +68,20 @@ def merge_app_settings(setting: str, original: Union[dict, list], deduplicate: b
                     raise TypeError("Only dict and list settings can be merged.")
 
 
+def lazy_config(key: str) -> Callable[[str], Any]:
+    """ Lazily get a config value from constance. Useful to bind constance
+    configs to other global settings to make them available to third-party
+    apps that are not aware of constance.
+    """
+
+    def _get_config(key: str) -> Any:
+        from constance import config  # noqa
+        return getattr(config, key)
+
+    # The type is guessed from the default value to improve lazy()'s behaviour
+    return lazy(_get_config, type(settings.CONSTANCE_CONFIG[key][0]))(key)
+
+
 def is_impersonate(request: HttpRequest) -> bool:
     if hasattr(request, "user"):
         return getattr(request.user, "is_impersonate", False)