Prepare for app factory [1 of 2] (#315)

* Move db, assets, debug toolbar and fix_paginate into nyaa.extensions
* Change all `from nyaa import db` imports to `from nyaa.extensions import db`
* Move `nyaa.torrents.create_magnet_from_es_info` context processor into template-utils blueprint
* Fix tools (wrap in `with app.app_context():` where needed)

db_migrate.py

@@ -1,17 +1,20 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 import sys
-from nyaa import app, db
+
 from flask_script import Manager
 from flask_migrate import Migrate, MigrateCommand
 
+from nyaa import app
+from nyaa.extensions import db
+
 migrate = Migrate(app, db)
 
 manager = Manager(app)
 manager.add_command("db", MigrateCommand)
 
 if __name__ == "__main__":
-	# Patch sys.argv to default to 'db'
-	sys.argv.insert(1, 'db')
+    # Patch sys.argv to default to 'db'
+    sys.argv.insert(1, 'db')
 
-	manager.run()
+    manager.run()