Improvements and fixes

Strings added by hand can now be deleted, and a network scan that finds nothing says why.

• Strings added by hand can be selected on the components page and deleted after a confirmation prompt, while detected strings stay.
• A network scan that finds nothing now says whether it could not reach the plant network or the scanned range held no devices.
• All address ranges of a direct remote-access connection are edited in one list and saved together; ranges already used on the plant are flagged.
• A plant's on-site agent keeps running when the plant is removed from an organization's remote access or that remote access is deleted.