Save special folder mapping and fetch folder list after creating account

[cketti]

To save special folders we use the same mechanism that is used when importing server settings. The server IDs of the special folders are saved in Account.imported*Folder. Then the first time the folder list is synced, SpecialFolderUpdater will map those server IDs to the database IDs for the folder and call Account.set*FolderId().

Closes #7329
Closes #5826

[wmontwe]

While this solves the issues mentioned, it adds complications:

The MessagingController is already huge (2.7k loc) and should be broken into smaller, more manageable modules. Where each module should have a single responsibility, making it easier to understand, maintain, and test.

Adding the MessagingController as direct dependency of AccountCreator, renders all efforts of making it testable obsolete/harder. It would help to introduce an interface that masks the complexity to the one necessary for account creation.

The special folder handling whitin the AccountCreator could also be moved to another class and tested individually.

The goal is not only to fix immediate issues but also to make the codebase more maintainable and adaptable for future changes. Remember, the key to successful refactoring is to balance immediate functional improvements with long-term code health. Idealy this is applied whenever code is touched. Each change should make the system a little more modular, understandable, and testable, aligning with the goal of a more maintainable and adaptable codebase.