Clean up gateway classes

Description

I would like to introduce some standard for gateway functions and successively update all gateway classes. There are some functions that perform more queries than necessary or that are too complex.

• Data that is not a primitive type (or an array of primitive types) should be returned as a DTO
• Functions that add data to the DB and return the id should throw an exception in case of an error. They should not return -1 to indicate errors.
• Functions that update an entry should not return anything and throw an exception in case of an error. It should be expected that they succeed. They should not return a boolean.
• Functions should not be too complex and should not execute too many queries. If a function contains logic or executes several queries, it should be split up and the logic can be moved into a transaction class. This will also remove unnecessary queries.
• Parameters and return values should not be false to indicate empty or missing data. They should either be null or an exception should be thrown.

And additionally some code style:

• Remove unused functions
• Add documentation to all functions

Classes

• AchievementGateway
• ActivityGateway
• ApplicationGateway
• BasketGateway
• BellGateway
• BlogGateway
• BuddyGateway
• BusinessCardGateway
• ContentGateway
• EventGateway
• FoodsaverGateway
• FoodSharePointGateway
• GroupGateway
• LegalGateway
• LoginGateway
• MailboxGateway
• MailsGateway
• MaintenanceGateway
• MapGateway
• MessageGateway
• PassportGeneratorGateway
• ProfileGateway
• PushNotificationGateway
• QuizGateway
• ForumGateway
• ForumFollowerGateway
• RegionGateway
• ReportGateway
• SearchGateway
• SettingsGateway
• StatsGateway
• StoreGateway
• StoreChainGateway
• TeamGateway
• UploadsGateway
• VotingGateway
• WallPostGateway
• WorkGroupGateway