Add FixFilePermissionsTask to fix up NULL file permissions from SS3

src/Dev/FixFilePermissionsTask.php

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+<?php
+
+namespace SilverStripe\Dev\Tasks;
+
+use SilverStripe\Assets\File;
+use SilverStripe\Dev\BuildTask;
+use SilverStripe\ORM\Queries\SQLUpdate;
+use SilverStripe\Security\InheritedPermissionFlusher;
+
+/**
+ * Files imported from SS3 might end up with broken permissions if there is a case conflict.
+ * This task resets the `CanViewType` of files that are `NULL`.
+ * You need to flush your cache after running this task via CLI.
+ */
+class FixFilePermissionsTask extends BuildTask
+{
+    private static $segment = 'FixFilePermissionsTask';
+
+    protected $title = 'Fix File Permissions';
+
+    protected $description = '
+    Reset the permission of files that have a `NULL` CanViewType value.
+    You need to flush your cache after running this task via CLI.
+    ';
+
+    public function run($request)
+    {
+        SQLUpdate::create(
+            File::singleton()->baseTable(),
+            ['CanViewType' => 'Inherit'],
+            ['ISNUll(CanViewType)']
+        )->execute();
+
+        // This part won't work if run from the CLI, because Apache and the CLI don't share the same cache.
+        InheritedPermissionFlusher::flush();
+
+        echo "Done!";
+    }
+}

tests/php/FixFilePermissionsTaskTest.php

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+<?php
+
+namespace SilverStripe\Assets\Tests;
+
+use SilverStripe\Assets\File;
+use SilverStripe\Assets\Folder;
+use SilverStripe\Assets\Image;
+use SilverStripe\Dev\SapphireTest;
+use SilverStripe\Dev\Tasks\FixFilePermissionsTask;
+
+/**
+ * Ensures that files with invalid permissions can be fixed.
+ */
+class FixFilePermissionsTaskTest extends SapphireTest
+{
+    protected static $fixture_file = 'FixFilePermissionsTaskTest.yml';
+
+    public function testTask()
+    {
+        $fixFilePermissionsTask = new FixFilePermissionsTask();
+        $fixFilePermissionsTask->run(null);
+
+        $this->assertEquals('Inherit', Folder::get()->where(['Name' => 'ParentFolder'])->first()->CanViewType);
+        $this->assertEquals('Inherit', Folder::get()->where(['Name' => 'SubFolder'])->first()->CanViewType);
+        $this->assertEquals('LoggedInUsers', Image::get()->where(['Name' => 'myimage.jpg'])->first()->CanViewType);
+        $this->assertEquals('OnlyTheseUsers', Image::get()->where(['Name' => 'myotherimage.jpg'])->first()->CanViewType);
+        $this->assertEquals('Inherit', File::get()->where(['Name' => 'anotherfile.jpg'])->first()->CanViewType);
+        $this->assertEquals('Inherit', File::get()->where(['Name' => 'file.jpg'])->first()->CanViewType);
+        $this->assertEquals('Anyone', File::get()->where(['Name' => 'picture.jpg'])->first()->CanViewType);
+        $this->assertEquals('Inherit', File::get()->where(['Name' => 'file2.jpg'])->first()->CanViewType);
+    }
+}

tests/php/FixFilePermissionsTaskTest.yml

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+SilverStripe\Assets\Folder:
+  parent:
+    Name: ParentFolder
+    CanViewType: NULL
+  subfolder:
+    Name: SubFolder
+    Parent: =>SilverStripe\Assets\Folder.parent
+SilverStripe\Assets\Image:
+  image1:
+    Name: myimage.jpg
+    CanViewType: LoggedInUsers
+  image2:
+    Name: myotherimage.jpg
+    ParentID: =>SilverStripe\Assets\Folder.subfolder
+    CanViewType: OnlyTheseUsers
+SilverStripe\Assets\File:
+  file1:
+    Name: anotherfile.jpg
+    CanViewType: Inherit
+  file2:
+    Name: file.jpg
+    ParentID: =>SilverStripe\Assets\Folder.parent
+    CanViewType: NULL
+  file3:
+    Name: picture.jpg
+    ParentID: =>SilverStripe\Assets\Folder.subfolder
+    CanViewType: Anyone
+  file4:
+    Name: file2.jpg
+    ParentID: =>SilverStripe\Assets\Folder.parent
+    CanViewType: