Draft: Mail settings reorganization

[kra-mo]

Note

This is a draft, here for reference and so that we can discuss.
Not to be implemented yet, not all library components required for the changes are available yet.

Reorganized settings using the new form components. See nextcloud/server#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.

"Always show images from" and "Internal addresses" items have menus with a single item called "Remove" with a trash can icon.