[WHIT-2788] Configurable document form objects

[jamiestamp]

What

This PR refactors the schema design for config-driven document types by separating concerns that were previously conflated under edit_screens. We now:

• removed settings.edit_screens, in favour of a new top-level forms hash. It describes things at the UI level: which fields appear on which forms.
• adds new top-level presenters field, describing which field values get sent to Publishing API (and how to map them)
• removes schema.properties in favour of schema.attributes: effectively a slimmed down version of the former.
• schema.validations remains unchanged, but we no longer have 'nested' validation (e.g. schema.properties.duration.validations) - the 'nested' validation has moved up into the schema.validations (in this example, schema.validations.duration). This isn't represented in the overview below as we only had one example and it's more of a side-effect than a deliberate change.

Overview of schema changes:

"schema": {
-  "properties": {
-    "body": {
-      "title": "Body",
-      "type": "string",
-      "format": "govspeak"
-    }
+  "attributes": {
+    "body": {
+      "type": "string",
+    }
+  }
}
+ "forms": {
+  "documents": {
+    "fields": {
+      "body": {
+        "title": "Body",
+        "description": "The main content of the page",
+        "block": "govspeak"
+      }
+    }
+  }
+ },
+ "presenters": {
+   "publishing_api": {
+    "body": "govspeak"
+  }
+}
"settings": {
-  "edit_screens": {
-   "document": ["body"]
- }

In order to not break anything, we're following the expand-and-contract methodology to support both the old and new schema kinds, branching our templates with conditional logic and duplicating our tests to exercise both.

Once merged, we'll migrate each standard edition config file in isolation, from the old format to the new. We'll then be able to delete the branching and the original tests.

Why

Bunching up properties into 'edit screens' was a good first step in #10786, allowing us to update different block content fields in isolation on separate tabs. But it was too restrictive: we had no way of denoting which fields should be 'grouped' (e.g. start & end date fields for an overall 'duration') and also we were having to wrestle with wanting to present the fields in one way but present them differently when presenting to Publishing API. The previous properties and edit_screens concepts were conflating too much.

The refactoring of the schemas we've done here should make it quicker and easier to iterate document types in future.

Additional notes

We have converted Topical Events to use the new forms schema. Once this is deployed and with sense checks done, we will migrate the other existing configurable document types.

Jira: https://gov-uk.atlassian.net/browse/WHIT-2788

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[ChrisBAshton]

Wow, the Render standard edition with a new schema format commit is quite hefty! Is there any way it can be broken up? Also that commit in general could do with a bit of a rewrite of the message (to rewrite confusing shorthand like "this includes st ed controller").

Currently I'm putting a lot of faith in the idea that tests have been copied over verbatim, with only the 'setup' code tweaked accordingly. It's a shame there's no way we can configure Minitest to just run the same exact existing tests twice but with two different setups. (Like RSpec's it_behaves_like de-duplicator?)

But as an overall approach, I like it 👍 good job, folks! 👏