Mail settings reorganization

Reorganized settings using the new form components. See https://github.com/nextcloud/server/issues/55667

"Trusted senders" has been renamed "Always show images from". As far as I can understand, this is the only thing that this feature does. Please correct me if I'm wrong.

<img alt="Image" src="https://github.com/user-attachments/assets/3107150d-5df2-467d-a045-8305ade5a441" />