Use native disk methods instead of the pseudo scoped disks

[LukeTowers]

WIP, requires wintercms/storm#141

[what-the-diff]

• Removed the cms.storage config and moved it to filesystems.php
• Added a new disk for each of uploads, media and resized files (scoped disks)
• Updated ImageResizer::getAvailableSources() to use scoped disks instead of cms storage config values
• Updated MediaLibrary::getStorageDisk() to use DISK constant instead of cms storage config value
  5-6: Moved getPublicPath(), getStorageDirectory(), isPublic() methods from File model into base class as they are used by other models that extend this one too (e9d8f7b1c). Also updated them so they work with scoped disks now rather than using the old CMS Storage configuration options which have been removed in favour or Laravel's Filesystem Adapter system now being used everywhere throughout Winter Storm codebase where possible/appropriate - see https://github.com/octobercms/winterstorm#filesystem-adapter for more information on how these changes affect you if at all! :)