Strip double / prefix when uploading to Media root directory

[Flynsarmy]

When uploading to the Media Manager's root directory, the media.file.upload event will prefix the $path variable with two / characters. This PR strips the first one.

Steps to Replicate:

• In any plugin add the following to your Plugin.php's boot method:

Event::listen('media.file.upload', function (\Backend\Widgets\MediaManager $mediaWidget, string $path, \Symfony\Component\HttpFoundation\File\UploadedFile $uploadedFile) {
    \Log::info($path);
});

• Go to Backend - Media section and upload an image to the root directory.
• Open /storage/logs/system.log and see the filepath logged was //myimage.jpg

The problem in depth

When uploading to the root directory, the following line produces a path of / because you're uploading to the Media Manager's root directory:

winter/modules/backend/traits/UploadableWidget.php

Line 81 in 86cfbfa

$path = !empty($this->uploadPath) ? $this->uploadPath : Request::input('path');

On the very next line we call validatePath:

winter/modules/backend/traits/UploadableWidget.php

Line 82 in 86cfbfa

$path = MediaLibrary::validatePath($path);

which strips the / suffix but then adds it back as a prefix:

winter/modules/system/classes/MediaLibrary.php

Line 471 in 86cfbfa

$path = '/'.trim($path, '/');

and finally on the next line, we add yet another / resulting in the behaviour we see in my issue:

winter/modules/backend/traits/UploadableWidget.php

Line 83 in 86cfbfa

$filePath = $path . '/' . $fileName;

This should result in uploads to the root directory of the Media Manager having a $path of /myimage.jpg as it should.

[LukeTowers]

Waiting for the other changes we discussed @Flynsarmy :)

[Flynsarmy]

I didn't include them in this PR because they're not part of the bug outlined. Would you prefer I tack them on here or put them in a separate PR that'd make more sense for them?

[LukeTowers]

I'd prefer they were included in the PR so we can make the single PR be about making the file.upload event actually useful rather than smaller pieces of the overall puzzle.