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/static/theme.scss

@@ -34,11 +34,11 @@
 // 1. Colors
 // ==========================================================================
 
-$primary-color: adjust-color(get-colour(get-theme-setting(colour_primary)), $alpha: 1);
+$primary-color: adjust-color(get-colour(get-setting(COLOUR_PRIMARY)), $alpha: 1);
 $primary-color-light: lighten($primary-color, 15%) !default;
 $primary-color-dark: darken($primary-color, 15%) !default;
 
-$secondary-color: adjust-color(get-colour(get-theme-setting(colour_secondary)), $alpha: 1);
+$secondary-color: adjust-color(get-colour(get-setting(COLOUR_SECONDARY)), $alpha: 1);
 $success-color: color("green", "base") !default;
 $error-color: color("red", "base") !default;
 $link-color: color("light-blue", "darken-1") !default;