fix: review of OutOfOffice settings form

[madbob]

Totally unrequested review of the OOO settings form, migrated to Nc* components.

Boldest changes:

• replacement of internal TextEditor with generic NcTextArea, applied because - after all - HTML editing was already disabled and it is not that different
• settings are hidden when "autorespond" is "off". The previous conditions to disable edit fields were, anyway, broken (this.enabled was tested as a boolean, while it is always a string), and my personal (and questionable!) opinion is that hiding unrequired components from the crowded settings panel helps in making it more readable

Fixes #9975

[kesselb]

Thanks, could you take a look at the conflicts?

replacement of internal TextEditor with generic NcTextArea, applied because - after all - HTML editing was already disabled and it is not that different

Newlines are not working anymore

[madbob]

I've preferred to entirely drop plaintext/HTML/plaintext conversion, and just use the bare version of the string.