[WIP] feat(migration): Improve mail import/export functionality

[DerDreschner]

Work in progress!

Overview

This pull request extends the previous MVP import/export functionality. The following things are now being migrated as well:

• Special mailboxes being set for mail accounts (includes migration of all mailboxes without their messages)
• Sieve settings being set for mail accounts
• Quick actions being created for mail accounts (includes migration of all tags)
• Imported S/MIME certificates and their relation to mail accounts and aliases
• Signatures being set only for specific aliases
• Defined internal addresses
• Defined trusted senders
• Text blocks, but only those not shared by any other group
• General app settings being set by the UI (e.g., view configuration)

Open tasks

• Improve the phpdoc @return array description regarding the mapping
• Discuss and decide what cases should be tested and how (see this comment)
• Implement the chosen testing solution (may happen in a separate PR)
• Squash commits
• Create user manual documentation

Open questions by myself

I was confused about the architecture. There are the mapper classes, which sometimes are being used via services. But sometimes they are used directly (examples: MailboxMapper, ActionStepMapper, TagMapper). When is it advised to create a service class instead of using the mapper directly? 🤔

Relates to #12001.