new applets app for cross-board usage of Applet() & UserApplet() models; dashboard migrations reset and apps reseeded w. new default specs; core.settings & many tests thru-out suite updated accordingly

src/apps/applets/admin.py

@@ -0,0 +1,11 @@
+from django.contrib import admin
+
+from apps.applets.models import Applet, UserApplet
+
+
+@admin.register(Applet)
+class AppletAdmin(admin.ModelAdmin):
+    list_display = ['slug', 'name', 'default_visible', 'grid_cols', 'grid_rows']
+    list_editable = ['grid_cols', 'grid_rows']
+
+admin.site.register(UserApplet)