Fail to start when using non-sqlite for snappea

See #252

This is not backwards compatible (it will now hard-break), but given snappea's
rather particular design decisions this is probably for the best, and given
what else was just discovered on #252 I strongly doubt anyone was actually using
it in such setups anyway.

snappea/__init__.py

@@ -67,3 +67,13 @@ def check_no_nested_settings_in_unnested_form(app_configs, **kwargs):
                 id="snappea.W001",
             ))
     return errors
+
+
+@register("snappea")
+def check_using_sqlite(app_configs, **kwargs):
+    if "sqlite" not in django_settings.DATABASES['snappea']['ENGINE']:  # covers both timed_sqlite_backend and sqlite3
+        return [Warning(
+            "The snappea 'database' backend (queue really) is not set to use SQLite. It should be.", id="snappea.W002",
+        )]
+
+    return []