Export and import for user migration

[ChristophWurst]

Feature Request

As a Nextcloud user I want to be able to export my data from Mail and import it on another instance.

Describe the solution you'd like

Implement a migrator: https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/user_migration.html#register-a-migrator

Work packages

• MVP implementation (name, email, mail server settings and aliases)
• Migrate special mailboxes
• Migrate account preferences
• Migrate sieve settings
• Migrate smime
• Migrate quick actions
• CI workflow to test re-import
• User manual documentation

Follow-ups

None

Out of scope

• Export of provisioned accounts (configuration owned by admin)
• Export of emails and related data (this data is stored on the IMAP server)
• Export of sieve scripts (this data is stored on the Sieve server)