Fix bug that removes advanced target options when editing backups

[Jojo-1000]

Closes #4553

The advanced options are stored in the URI. Since a non-standard format with backslashes is used for windows paths, parsing these options fails when editing a backup with a file target in the web UI.

This change makes an exception for file URIs to allows those paths.

Steps to reproduce:

• Create backup with local file target on Windows
• Specify advanced options on the target page (Step 2), e.g. disable-nagling
• Save backup
• Open backup to edit and go to Step 2

Current behavior:

• The advanced options are cleared (also applies to username and password) and remain cleared if the backup is saved

New behavior:

• The advanced options remain the same

Alternative steps to reproduce:

• Create backup with local file target on Windows
• Specify advanced options on Step 2
• Export target URI to Clipboard (... menu on Backup destination)
  or use example URI: file://C:\temp\?auth-username=abc&auth-password=def&disable-nagling=true
• Import the target URI on the same page

Current behavior:

• Only the path is imported, advanced options are cleared

New behavior:

• All options are imported correctly and remain the same

[duplicatibot]

This pull request has been mentioned on Duplicati. There might be relevant details there:

https://forum.duplicati.com/t/auth-warnings-and-too-small-backupfile/16422/5

[duplicatibot]

This pull request has been mentioned on Duplicati. There might be relevant details there:

https://forum.duplicati.com/t/backups-fails-likely-connected-to-400mb-video-files/16439/7

[duplicatibot]

This pull request has been mentioned on Duplicati. There might be relevant details there:

https://forum.duplicati.com/t/consult-double-copy-send-email/16448/2

[duplicatibot]

This pull request has been mentioned on Duplicati. There might be relevant details there:

https://forum.duplicati.com/t/unable-to-read-data-from-the-transport-connection-the-socket-has-been-shut-down/14860/6