Towards rationalising settings form & preferences form (partial of 12731)

[eileenmcnaughton]

Overview

Adds a trait class & starts to align Settings form with preferences form

Before

The settings form & the preferences form do the same thing but evolved separately. Settings uses metadata, preferences doesn't.

After

This adds a trait that can be shared & the parts of both that use metadata can use the shared section

Technical Details

Over time we should remove both forms in favour of a new clean one based on the trait. I think the trait would be merged into that form.

Note that one issue TBC is how to define 'serialize' on the form. In the DAO we use a string but the generator converts it to a constant. I used just a constant here.

I can break this down into some smaller commits.

Comments
This is a cut down of #12731 for easier review

[civibot]

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[eileenmcnaughton]

moved to the trait

[eileenmcnaughton]

test this please

[colemanw]

Looks simple and safe.