Replace django-dbsettings with django-constance

Rationale: django-constance is a bit better maintained and has some features we need, e.g. injecting config into templates through a context processor. Also, decoupling the settings definitions from import time allows us to implement lazy access to the config, so we can later access them in other variables insettings.py directly.

aleksis/core/util/sass_helpers.py

+from django.conf import settings
+
 from colour import web2hex
+from constance import config
 from sass import SassColor
 
-from aleksis.core.db_settings import theme_settings
-
 
 def get_colour(html_colour: str) -> SassColor:
     rgb = web2hex(html_colour, force_long=True)[1:]
@@ -11,5 +12,5 @@ def get_colour(html_colour: str) -> SassColor:
     return SassColor(r, g, b, 255)
 
 
-def get_theme_setting(setting: str) -> str:
-    return getattr(theme_settings, setting, "")
+def get_setting(setting: str) -> str:
+    return getattr(config, setting, "") or getattr(settings, setting, "")