Default "Content Author" group does not have file edit permissions

[chillu]

In 3.x, access to the assets section (CMS_Access_AssetAdmin) or the CMS (CMS_Access_LeftAndMain) was enough to edit files. Which meant the default "Content Author" group could edit all files, which makes sense. See https://github.com/silverstripe/silverstripe-framework/blob/3/filesystem/File.php#L338

In 4.x, we've significantly increased the complexity of the file permission model (see file security makes no sense and docs update.

Editing files now requires the FILE_EDIT_ALL permission, unless other permissions are set (e.g. giving access to certain groups). I consider this a regression from 3.x, and a default that doesn't make any sense. It leads to further confusions like @kinglozzer looking at the ownership of a file for permission control (comment).

We should mention this in the 4.0 and 4.5 upgrading guide, and change the default Group->requireDefaultRecords() to add this permission for new installs.

/cc @tractorcow @sminnee @dnsl48