NEW Make CMSFields scaffolding configurable, plus new options

[GuySartorelli]

Doing this to support the CMS 6 changes - but there's no reason for this new feature to wait until April since it's not BC breaking.

• Adds three new options for FormScaffolder:
  • tabsOnly: Only return the Root.Main tab in the FieldList (effectively don't scaffold anything).
    • This isn't strictly necessary, but it does make it easier to follow best practice (calling parent::getCMSFields()) even in scenarios where you want to build all your form fields from scratch.
  • ignoreFields: Array of field names in $db or relations in $has_one which should not be scaffolded.
  • ignoreRelations: Array of relations in $has_many or $many_many which should not be scaffolded.
  • restrictRelations: Array of relation names to use as an allow list. Similar to the existing restrictFields
• Makes the options used to scaffold fields in getCMSFields() configurable.

Note

ignoreFields includes $has_one to match the behaviour of both restrictFields (includes $has_one) and includeRelations (for $has_many and $many_many only)

Issue

• Scaffold fields for SiteTree silverstripe-cms#2767

[GuySartorelli]

This isn't strictly needed, but it makes the test easier.
It seemed silly to be throwing an exception when $currentPointer is null, because in this scenario the tab doesn't exist so we can just return null from the method.

[GuySartorelli]

This was intentionally changed to not include has_one in 524d7a9

[GuySartorelli]

This isn't actually used at all

[GuySartorelli]

Added restrictRelations option which mirrors restrictFields option. This will facilitate easier upgrades for people who just want to use the old behaviour instead of updating the getCMSFields() and updateCMSFields() implementations in CMS 6, as they can just allow things to scaffold for SiteTree and other third-party pages, and leave their own pages as they are.