FIX: Remove legacy fields which prevent page publish (fixes #2455)

[kinglozzer]

Description

This updates the list of fields which are ignored when deciding whether a new version (and therefore a new live version) should be created. Also made it configurable while I was there.

Manual testing steps

Create a page type with a field Status, save and publish. Then change only the status field and publish again: a new version won’t be written, and the live site won’t update. If you change status and something else, it will.

Issues

• SiteTree won't write new versions if only a field named 'Status' is changed #2455

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[kinglozzer]

HasBrokenFile and HasBrokenLink are the only two fields that actually exist anymore.

I thought about changing VersionID to Version (which is what the field is now named), but that would be a behavioural change. It would also be a bit weird: changing the version number in the SiteTree table without writing a version would mean that SiteTree and SiteTree_Versions could end up out of sync.

[GuySartorelli]

Overall looks good, thanks!

Can you please retarget this to 5.2? That branch is still supported for bug fixes so it'd be good to get the fix in there instead of having to wait for 5.3.0 stable to release.

[GuySartorelli]

LGTM, works well locally.