interfaces: convert settings page to MVC/API

[fichtner]

Important notices

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

The page is relatively easy and contained, but settings need to be migrated. Test out this page before going to more complex settings pages.

We need our batch of MVC/API conversions for this iteration because even though it's over 80% done that's still a bit left. :)

[fichtner]

49bb682 should be taken into account when adding the model