FIX: ensure children keep the same visibility when writing folder

[sunnysideup]

Description

When you write a folder (in my case I was setting up default folders using the requireDefaultRecords method), it also write all its children.

For some reason, when you extend the File (e.g. Document extends File), this start behaving differently. With this extension, if a file is in a folder, and you write the folder, the file becomes "protected" (or at least modified on draft).

The code is here:
https://github.com/silverstripe/silverstripe-assets/blob/2/src/Folder.php#L296-L298

This is called from onAfterWrite and also from onAfterSkippedWrite (akin to an onBeforeWrite).

Manual testing steps

1. install Silverstripe
2. extend File like this: class Document extends File
3. Add at least one DB field to Document (e.g. Content => 'HTMLText")
4. Upload a file in the folder. Change the file class to be Document (e.g. using onBeforeWrite).
5. save the parent folder
6. Check the versioned status of the file, it should now be "changed" to draft.

Issues

Here are a couple of issues that may relate:

• Very poor publish/delete file performance in large folder #681
• Files with access restriction in Draft are restricted on Live #385
• protect assets still have hash in the file link, and the old file link doesn't redirect to the new file  #304

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