refactor: Migrate to API v2

- Added auto schema migration
- Refactored /api/subs, /api/collections, /api/artifacts. Now these APIs will return array instead of object. This enables sorting items in the future.

backend/src/constants.js

@@ -0,0 +1,9 @@
+export const SCHEMA_VERSION_KEY = 'schemaVersion';
+export const SETTINGS_KEY = 'settings';
+export const SUBS_KEY = 'subs';
+export const COLLECTIONS_KEY = 'collections';
+export const ARTIFACTS_KEY = 'artifacts';
+export const RULES_KEY = 'rules';
+export const GIST_BACKUP_KEY = 'Auto Generated Sub-Store Backup';
+export const GIST_BACKUP_FILE_NAME = 'Sub-Store';
+export const ARTIFACT_REPOSITORY_KEY = 'Sub-Store Artifacts Repository';