Add FixFilePermissionsTask to fix up NULL file permissions from SS3

[bergice]

This just resets the view enum to Inherit for records that have it set to NULL.

This task will be combined with other file migration tasks at some point so we don't have too many of them floating around.

This should also just be necessary after upgrading from SS3 as it is a datafix.

[chillu]

This should be a subtask of MigrateFileTask, for the same reasons as thumbnail and shortcode migration subtasks. This is part of the overall "4.x file migration", but in some cases you just want to run those tasks in isolation. I don't want people doing 3.x -> 4.x upgrades to start off with running a whole bunch of arbitrary CLI commands to get started :) Can you please rewrite this to be a subtask, based on the work in #235

[chillu]

See comment above :)

[bergice]

This should be a subtask of MigrateFileTask, for the same reasons as thumbnail and shortcode migration subtasks. This is part of the overall "4.x file migration", but in some cases you just want to run those tasks in isolation. I don't want people doing 3.x -> 4.x upgrades to start off with running a whole bunch of arbitrary CLI commands to get started :) Can you please rewrite this to be a subtask, based on the work in #235

@chillu should we merge https://github.com/silverstripe/silverstripe-framework/pull/8924/files first or do you want me to cherry-pick/rebase on your branch?

[chillu]

I'd say rebase on that PR, rather than cherry pick. Don't let the silverstripe/silverstripe-framework#8924 merge block you for now.

Rebased