Autoscaffold DataObject in CMS 6

[maxime-rainville]

We'll add the ability to auto-scaffold field in CMS 5.3. However no native DataObjects will be using it.

Acceptance criteria

• Link autoscaffold to LinkField and MultiLinkField
• When asset-admin is installed, File autoscaffold to UploadField for has_many and many_many, with multi-upload set to true
• SiteTree autoscaffold to TreeDropdownField and TreeMultiDropdownField
• ElementalArea HAS_ONE autoscaffolds ElementalAreaField and ElementalAreasExtension is refactored accordingly.
• EditableFormField scaffold a custom gridifeld
• TaxonomyTerm autoscaffolded to searchabledropownfield and searchablemultidropdownfield
• Any other DataObjects that could be sensibly autoscaffolded is updated to auto-scafolled
• Any getCMSFields or updateCMSFields methods that manually replaces a FormField with one that could be auto-scaffolded is updated to do so.
• maybe all classes that use Hierarchy other than mentioned above autoscaffold to TreeDropdownField and TreeMultiDropdownField

Post-implementation notes

• It's not feasible to change the ElementalArea to auto-scaffold in an ElementalAreaField, because the ElementalAreasExtension is still required for it to work as expected (and to exclude it when it should be excluded, and update various state for the field, etc). Auto-scaffolding as ElementalAreaField would just be a bad dev experience.
• I opted to not make the various models in silverstripe/userforms scaffoldable because it requires a lot of refactoring for very little benefit. Outside of the models in that module it's unlikely people will be having direct relations to those models in their projects.
• I've added autoscaffolding for BlogTag and BlogCategory to match how BlogPost sets them up - however, I can't update BlogPost to take advantage of that just yet. That'll be handled in Scaffold fields for SiteTree silverstripe-cms#2767

PRs

Kitchen sink CI run - note that because there's a fork for framework, we expect to see a failure in recipe-reporting-tools. There's a test in there that cannot pass when a forked framework is used.

• ENH Take advantage of auto-scaffolded form fields silverstripe-subsites#581
• ENH Auto-scaffold blog categories and tags formfields silverstripe-blog#769
• ENH Auto-scaffold SiteTree relations with tree fields silverstripe-cms#2970
• ENH Use better auto-scaffolded form fields silverstripe-linkfield#301
• ENH Auto-scaffold Member and Group with appropriate form fields silverstripe-framework#11285
• ENH Take advantage of auto-scaffolded formfields for link silverstripe-frameworktest#190
• ENH Scaffold TaxonomyTerm with searchable dropdown silverstripe-taxonomy#119
• ENH Use appropriate formfields when auto-scaffolding files silverstripe-assets#618
• DOC Document changes to scaffolded fields developer-docs#540