Replace Filesystem, FTP with FlySystem in order to support more filesystems

首先定位 issue 中提到的自定义 FileSystem 和 FTP 模块，然后将它们的 API 与描述中链接的 FlySystem API 进行比较。在决定迁移范围之前，梳理现有模块的行为和支持的操作；当替换实现支持所需的操作、不会丢失当前功能，并且模块测试通过时，即视为完成。

"FlySystem is a filesystem abstraction which allows you to easily swap out a local filesystem for a remote one. Reducing technical debt and chance of vendor lock-in."

It could replace the custom FileSystem and FTP modules and offer support for many more filesystems. Here is a list of what's available today :

• Local
• Azure
• AWS S3 V2
• AWS S3 V3
• Copy.com
• Dropbox
• FTP / SFTP
• GridFS
• Memory
• Rackspace
• WebDAV
• PHPCR
• ZipArchive

The API

http://flysystem.thephpleague.com/api/

Write Files

$filesystem->write('path/to/file.txt', 'contents');

Update Files

$filesystem->update('path/to/file.txt', 'new contents');

Write or Update Files

$filesystem->put('path/to/file.txt', 'contents');

Read Files

$contents = $filesystem->read('path/to/file.txt');

Check if a file exists

$exists = $filesystem->has('path/to/file.txt');

Delete Files

$filesystem->delete('path/to/file.txt');

Read and Delete

$contents = $filesystem->readAndDelete('path/to/file.txt');

Rename Files

$filesystem->rename('filename.txt', 'newname.txt');

Copy Files

$filesystem->copy('filename.txt', 'duplicate.txt');

Get Mimetypes

$mimetype = $filesystem->getMimetype('path/to/file.txt');

Get Timestamps

$timestamp = $filesystem->getTimestamp('path/to/file.txt');

Get File Sizes

$size = $filesystem->getSize('path/to/file.txt');

Create Directories

$filesystem->createDir('path/to/nested/directory');

Directories are also made implicitly when writing to a deeper path

$filesystem->write('path/to/file.txt', 'contents');

Delete Directories

$filesystem->deleteDir('path/to/directory');