Add image support to the text editor: Use editor mode for signature editor

[kesselb]

Is your feature request related to a problem? Please describe.

Follow up https://github.com/nextcloud/mail/pull/6935

TextEditor for signature does not respect the configured editor mode and always uses richtext mode.

Describe the solution you'd like

Set editor mode for the signature text editor. Warn user when changing from html to text (loosing the format/images).

Describe alternatives you've considered

No response

Additional context

No response